Packages/ICCeryCore/Sources/ICCeryCore/Logging/AppLogger.swift

@@ -15,6 +15,7 @@ public enum LogLevel: String, Codable, Sendable, CaseIterable {
         }
     }
 
+    /// Lower rank = more severe. `shouldLog` keeps `rank <= min`.
     var rank: Int {
         switch self {
         case .error: return 0
@@ -26,16 +27,19 @@ public enum LogLevel: String, Codable, Sendable, CaseIterable {
     }
 }
 
-/// Central logger. For M1 PR2 this writes to `os.Logger` only;
-/// issue #5 adds the rolling file sink and runtime `setLevel`.
+/// Central logger: `os.Logger` + rolling file sink (`LogSink`), level
+/// gated at write time so a settings save takes effect immediately
+/// (#158).
 public struct AppLogger: Sendable {
     public static let shared = AppLogger(category: "app")
 
     private let osLog: Logger
+    private let sink: LogSink
     public let category: String
 
-    public init(category: String) {
+    public init(category: String, sink: LogSink = .shared) {
         self.category = category
+        self.sink = sink
         self.osLog = Logger(
             subsystem: AppPaths.bundleIdentifier,
             category: category
@@ -44,7 +48,10 @@ public struct AppLogger: Sendable {
 
     public func log(_ level: LogLevel, _ message: @autoclosure () -> String) {
         let text = LogSanitizer.sanitize(message())
-        osLog.log(level: level.osType, "\(text, privacy: .public)")
+        if level.rank <= sink.level.rank {
+            osLog.log(level: level.osType, "\(text, privacy: .public)")
+        }
+        sink.write(level: level, category: category, message: text)
     }
 
     public func error(_ message: @autoclosure () -> String) { log(.error, message()) }

Packages/ICCeryCore/Sources/ICCeryCore/Logging/RollingFileLogger.swift

@@ -0,0 +1,127 @@
+import Foundation
+import OSLog
+
+/// Rolling file sink for `AppLogger` — `~/Library/Logs/<bundle>/
+/// iccery.log`, rotated at 5 MiB, keeping 5 historical segments
+/// (`iccery.log.1` … `iccery.log.5`).
+///
+/// The minimum level is **runtime state** (#158): `setLevel` takes
+/// effect immediately — at startup and on every settings save.
+public final class LogSink: @unchecked Sendable {
+
+    public static let shared = LogSink(fileURL: AppPaths.logFile)
+
+    private let lock = NSLock()
+    private let fileURL: URL
+    private var minimumLevel: LogLevel
+    private var handle: FileHandle?
+
+    /// 5 MiB per segment, 5 historical segments kept.
+    public static let maxSegmentBytes: UInt64 = 5 * 1024 * 1024
+    public static let keptSegments = 5
+
+    public init(
+        fileURL: URL = AppPaths.logFile,
+        minimumLevel: LogLevel? = nil
+    ) {
+        self.fileURL = fileURL
+        #if DEBUG
+        self.minimumLevel = minimumLevel ?? .debug
+        #else
+        self.minimumLevel = minimumLevel ?? .info
+        #endif
+    }
+
+    public var level: LogLevel {
+        lock.lock()
+        defer { lock.unlock() }
+        return minimumLevel
+    }
+
+    /// Applied at startup AND on every settings save (issue #5, #158).
+    public func setLevel(_ level: LogLevel) {
+        lock.lock()
+        minimumLevel = level
+        lock.unlock()
+    }
+
+    /// `nil` → DEBUG-build default (.debug) / release (.info).
+    public func applySettings(_ settings: AppSettings) {
+        setLevel(settings.effectiveLogLevel)
+    }
+
+    public func shouldLog(_ level: LogLevel) -> Bool {
+        level.rank <= { lock.lock(); defer { lock.unlock() }; return minimumLevel }().rank
+    }
+
+    // MARK: - Writing
+
+    /// Appends a `YYYY-MM-DD HH:mm:ss.SSS [LEVEL] category: msg` line,
+    /// rotating first when the active segment exceeds 5 MiB.
+    public func write(level: LogLevel, category: String, message: String) {
+        guard shouldLog(level) else { return }
+        lock.lock()
+        defer { lock.unlock() }
+        rotateIfNeeded()
+        openIfNeeded()
+        let stamp = Self.timestamp()
+        let line = "\(stamp) [\(level.rawValue.uppercased())] \(category): \(message)\n"
+        if let data = line.data(using: .utf8) {
+            handle?.write(data)
+        }
+    }
+
+    private static let formatter: DateFormatter = {
+        let f = DateFormatter()
+        f.dateFormat = "yyyy-MM-dd HH:mm:ss.SSS"
+        f.locale = Locale(identifier: "en_US_POSIX")
+        return f
+    }()
+
+    private static func timestamp() -> String {
+        formatter.string(from: Date())
+    }
+
+    private func openIfNeeded() {
+        guard handle == nil else { return }
+        try? FileManager.default.createDirectory(
+            at: fileURL.deletingLastPathComponent(), withIntermediateDirectories: true
+        )
+        if !FileManager.default.fileExists(atPath: fileURL.path) {
+            FileManager.default.createFile(atPath: fileURL.path, contents: nil)
+        }
+        handle = try? FileHandle(forWritingTo: fileURL)
+        try? handle?.seekToEnd()
+    }
+
+    /// Shifts `iccery.log.4→.5`, `.3→.4`, …, `.log→.1` and resets the
+    /// writer. Oldest segment is deleted.
+    private func rotateIfNeeded() {
+        guard FileManager.default.fileExists(atPath: fileURL.path),
+              let attrs = try? FileManager.default.attributesOfItem(atPath: fileURL.path),
+              let size = attrs[.size] as? UInt64,
+              size >= Self.maxSegmentBytes
+        else { return }
+
+        try? handle?.close()
+        handle = nil
+        let fm = FileManager.default
+        let oldest = fileURL.appendingPathExtension("\(Self.keptSegments)")
+        try? fm.removeItem(at: oldest)
+        for i in stride(from: Self.keptSegments - 1, through: 1, by: -1) {
+            let src = fileURL.appendingPathExtension("\(i)")
+            let dst = fileURL.appendingPathExtension("\(i + 1)")
+            if fm.fileExists(atPath: src.path) {
+                try? fm.moveItem(at: src, to: dst)
+            }
+        }
+        try? fm.moveItem(at: fileURL, to: fileURL.appendingPathExtension("1"))
+    }
+
+    /// Tail of the active log for the settings dialog's "copy excerpt".
+    public func tailExcerpt(maxBytes: Int = 32 * 1024) -> String {
+        guard let data = try? Data(contentsOf: fileURL) else { return "" }
+        let slice = data.suffix(maxBytes)
+        return String(decoding: slice, as: UTF8.self)
+    }
+}

Packages/ICCeryCore/Sources/ICCeryCore/Settings/AppSettings.swift

@@ -0,0 +1,115 @@
+import Foundation
+
+/// A saved wizard preset slot (docs/22 §Presets). The preset *engine*
+/// lands in issue #11; for M1 the store only needs a Codable container.
+public struct CustomPreset: Codable, Equatable, Sendable {
+    public var name: String
+    /// Opaque per-stage form values — keyed by field id.
+    public var values: [String: String]
+
+    public init(name: String, values: [String: String] = [:]) {
+        self.name = name
+        self.values = values
+    }
+}
+
+/// Where `install_profile` drops finished profiles (docs/22).
+public enum InstallLocation: String, Codable, Sendable, CaseIterable {
+    case user
+    case system
+}
+
+/// `settings.json` model (docs/22). snake_case keys match the v1 file
+/// so field names stay identical across rewrites.
+public struct AppSettings: Codable, Equatable, Sendable {
+
+    /// User override for Argyll binaries; `nil` → bundled sidecars.
+    public var argyllBinaryDir: String?
+
+    /// Stored but **never applied to argv** — Stage 2's own instrument
+    /// select is the live source (docs/04 §0.1).
+    public var defaultInstrument: String?
+
+    /// `nil` → `.debug` in debug builds, `.info` in release (#158).
+    public var logLevel: LogLevel?
+
+    public var deltaEGoodMax: Double
+    public var deltaEWarningMax: Double
+    public var customPresets: [CustomPreset]
+    public var enableI1Pro2Leds: Bool
+    public var calibrationStaleDays: Int
+    public var defaultInstallLocation: InstallLocation
+    public var askBeforeOverwriteProfile: Bool
+    public var openColorPanelAfterInstall: Bool
+
+    public init(
+        argyllBinaryDir: String? = nil,
+        defaultInstrument: String? = nil,
+        logLevel: LogLevel? = nil,
+        deltaEGoodMax: Double = 2.0,
+        deltaEWarningMax: Double = 5.0,
+        customPresets: [CustomPreset] = [],
+        enableI1Pro2Leds: Bool = false,
+        calibrationStaleDays: Int = 30,
+        defaultInstallLocation: InstallLocation = .user,
+        askBeforeOverwriteProfile: Bool = true,
+        openColorPanelAfterInstall: Bool = false
+    ) {
+        self.argyllBinaryDir = argyllBinaryDir
+        self.defaultInstrument = defaultInstrument
+        self.logLevel = logLevel
+        self.deltaEGoodMax = deltaEGoodMax
+        self.deltaEWarningMax = deltaEWarningMax
+        self.customPresets = customPresets
+        self.enableI1Pro2Leds = enableI1Pro2Leds
+        self.calibrationStaleDays = calibrationStaleDays
+        self.defaultInstallLocation = defaultInstallLocation
+        self.askBeforeOverwriteProfile = askBeforeOverwriteProfile
+        self.openColorPanelAfterInstall = openColorPanelAfterInstall
+    }
+
+    public static let `default` = AppSettings()
+
+    /// Effective log level — runtime state, not just persistence (#158).
+    public var effectiveLogLevel: LogLevel {
+        if let logLevel { return logLevel }
+        #if DEBUG
+        return .debug
+        #else
+        return .info
+        #endif
+    }
+
+    enum CodingKeys: String, CodingKey {
+        case argyllBinaryDir = "argyll_binary_dir"
+        case defaultInstrument = "default_instrument"
+        case logLevel = "log_level"
+        case deltaEGoodMax = "delta_e_good_max"
+        case deltaEWarningMax = "delta_e_warning_max"
+        case customPresets = "custom_presets"
+        case enableI1Pro2Leds = "enable_i1pro2_leds"
+        case calibrationStaleDays = "calibration_stale_days"
+        case defaultInstallLocation = "default_install_location"
+        case askBeforeOverwriteProfile = "ask_before_overwrite_profile"
+        case openColorPanelAfterInstall = "open_color_panel_after_install"
+    }
+
+    /// UI-facing validation. Strings are part of the contract (issue #5).
+    public static let errorNegativeDeltaE = "ΔE thresholds cannot be negative."
+    public static let errorThresholdOrder =
+        "Good ΔE threshold must be strictly less than the warning threshold."
+
+    /// All validation errors, in declaration order. Empty = valid.
+    public func validate() -> [String] {
+        var errors: [String] = []
+        if deltaEGoodMax < 0 || deltaEWarningMax < 0 {
+            errors.append(Self.errorNegativeDeltaE)
+        }
+        if deltaEGoodMax >= deltaEWarningMax {
+            errors.append(Self.errorThresholdOrder)
+        }
+        return errors
+    }
+
+    public var isValid: Bool { validate().isEmpty }
+}

Packages/ICCeryCore/Sources/ICCeryCore/Settings/SettingsStore.swift

@@ -0,0 +1,47 @@
+import Foundation
+
+/// Persists `AppSettings` to
+/// `~/Library/Application Support/com.gronod.iccery2/settings.json`
+/// (issue #5 — the v1 path is never read).
+///
+/// Writes are atomic (`AtomicFileWriter`). Invalid/corrupt JSON falls
+/// back to defaults. Saving posts `settingsDidChange` so #20 can
+/// reclassify swatches.
+public final class SettingsStore: Sendable {
+
+    /// Posted on `NotificationCenter.default` after every successful save.
+    public static let settingsDidChange =
+        Notification.Name("com.gronod.iccery2.settingsDidChange")
+
+    public let fileURL: URL
+
+    public init(fileURL: URL = AppPaths.appDataDir.appendingPathComponent("settings.json")) {
+        self.fileURL = fileURL
+    }
+
+    public func load() -> AppSettings {
+        guard let data = try? Data(contentsOf: fileURL),
+              let settings = try? JSONDecoder().decode(AppSettings.self, from: data)
+        else {
+            return .default
+        }
+        return settings
+    }
+
+    /// Validates before persisting — throws `SettingsError` listing
+    /// every violation; nothing is written on failure.
+    public func save(_ settings: AppSettings) throws {
+        let errors = settings.validate()
+        guard errors.isEmpty else {
+            throw SettingsError.validationFailed(errors)
+        }
+        let encoder = JSONEncoder()
+        encoder.outputFormatting = [.prettyPrinted, .sortedKeys]
+        try AtomicFileWriter.write(encoder.encode(settings), to: fileURL)
+        NotificationCenter.default.post(name: Self.settingsDidChange, object: nil)
+    }
+
+    public enum SettingsError: Error, Equatable {
+        case validationFailed([String])
+    }
+}

Sources/ICCery/ICCeryApp.swift

@@ -7,6 +7,13 @@ struct ICCeryApp: App {
     @NSApplicationDelegateAdaptor(AppDelegate.self) private var appDelegate
     @State private var model = WizardViewModel()
 
+    init() {
+        try? AppPaths.ensureDirectories()
+        // Log level is runtime state — apply persisted settings at
+        // startup (#158); the Settings sheet re-applies on save.
+        LogSink.shared.applySettings(SettingsStore().load())
+    }
+
     var body: some Scene {
         // Single fixed window (docs/21 §Shell: 1280×800, min 1100×700).
         Window("ICCery", id: "main") {

Sources/ICCery/RootView.swift

@@ -29,15 +29,7 @@ struct RootView: View {
         .frame(minWidth: 1100, minHeight: 700)
         .background(Theme.background)
         .sheet(isPresented: $showingSettings) {
-            // Full settings dialog lands in issue #5.
-            VStack(spacing: 12) {
-                Text("Settings").font(.headline)
-                Text("Implemented in issue #5.")
-                    .foregroundStyle(.secondary)
-                Button("Close") { showingSettings = false }
-            }
-            .padding(24)
-            .frame(width: 420)
+            SettingsView()
         }
         .alert("ICCery 2.0.0", isPresented: $showingAbout) {
             Button("OK") {}

Sources/ICCery/SettingsView.swift

@@ -0,0 +1,169 @@
+import SwiftUI
+import ICCeryCore
+
+/// Settings sheet (issue #5, docs/21 §Settings). Dark-theme Form with
+/// the full v1 field set; ΔE validation shows inline under the fields.
+struct SettingsView: View {
+    @State var model = SettingsViewModel()
+    @Environment(\.dismiss) private var dismiss
+
+    private static let instruments: [(code: String, label: String)] = [
+        ("i1", "X-Rite i1Pro / i1Pro 2"),
+        ("p3", "X-Rite i1Pro 3 / 3 Plus"),
+        ("CM", "ColorMunki"),
+        ("SS", "Specbos / Spectraval"),
+        ("20", "Gretag i1Display 2"),
+        ("22", "X-Rite i1Display Pro / ColorMunki Display"),
+        ("41", "Datacolor Spyder 4/5"),
+        ("51", "Spyder X"),
+    ]
+
+    var body: some View {
+        VStack(spacing: 0) {
+            Form {
+                Section("Argyll") {
+                    HStack {
+                        TextField(
+                            "Bundled sidecars",
+                            text: Binding(
+                                get: { model.settings.argyllBinaryDir ?? "" },
+                                set: {
+                                    model.settings.argyllBinaryDir =
+                                        $0.isEmpty ? nil : $0
+                                }
+                            )
+                        )
+                        Button("Browse…") {
+                            if let dir = FileDialogService.shared.selectDirectory() {
+                                model.settings.argyllBinaryDir = dir.path
+                            }
+                        }
+                    }
+                    Text("Leave empty to use the bundled Argyll tools.")
+                        .font(.caption)
+                        .foregroundStyle(.secondary)
+
+                    Picker(
+                        "Default instrument",
+                        selection: Binding(
+                            get: { model.settings.defaultInstrument ?? "" },
+                            set: {
+                                model.settings.defaultInstrument =
+                                    $0.isEmpty ? nil : $0
+                            }
+                        )
+                    ) {
+                        Text("None").tag("")
+                        ForEach(Self.instruments, id: \.code) {
+                            Text($0.label).tag($0.code)
+                        }
+                    }
+                    Text("Display-only — Stage 2's instrument select is used for actual runs.")
+                        .font(.caption)
+                        .foregroundStyle(.secondary)
+
+                    Toggle(
+                        "Enable i1Pro 2 LEDs",
+                        isOn: $model.settings.enableI1Pro2Leds
+                    )
+                }
+
+                Section("Verification") {
+                    HStack {
+                        Text("Good ΔE ≤")
+                        TextField(
+                            "2.0",
+                            value: $model.settings.deltaEGoodMax,
+                            format: .number
+                        )
+                        .frame(width: 60)
+                        Text("Warning ΔE ≤")
+                        TextField(
+                            "5.0",
+                            value: $model.settings.deltaEWarningMax,
+                            format: .number
+                        )
+                        .frame(width: 60)
+                    }
+                    ForEach(model.validationErrors, id: \.self) { error in
+                        Text(error)
+                            .font(.caption)
+                            .foregroundStyle(.red)
+                    }
+                }
+
+                Section("Calibration") {
+                    HStack {
+                        Text("Stale after")
+                        TextField(
+                            "30",
+                            value: $model.settings.calibrationStaleDays,
+                            format: .number
+                        )
+                        .frame(width: 60)
+                        Text("days")
+                    }
+                }
+
+                Section("Profile install") {
+                    Picker(
+                        "Install location",
+                        selection: $model.settings.defaultInstallLocation
+                    ) {
+                        Text("User library").tag(InstallLocation.user)
+                        Text("System library").tag(InstallLocation.system)
+                    }
+                    Toggle(
+                        "Ask before overwriting a profile",
+                        isOn: $model.settings.askBeforeOverwriteProfile
+                    )
+                    Toggle(
+                        "Open ColorSync after install",
+                        isOn: $model.settings.openColorPanelAfterInstall
+                    )
+                }
+
+                Section("Logging") {
+                    Picker(
+                        "Log level",
+                        selection: Binding(
+                            get: { model.settings.logLevel },
+                            set: { model.settings.logLevel = $0 }
+                        )
+                    ) {
+                        Text("Default").tag(LogLevel?.none)
+                        ForEach(LogLevel.allCases, id: \.self) {
+                            Text($0.rawValue.capitalized).tag(LogLevel?.some($0))
+                        }
+                    }
+                    HStack {
+                        Button("Open log folder") { model.openLogFolder() }
+                        Button("Copy path") { model.copyLogPath() }
+                        Button("Copy excerpt") { model.copyLogExcerpt() }
+                    }
+                }
+            }
+            .formStyle(.grouped)
+
+            Divider()
+
+            HStack {
+                if model.savedFlash {
+                    Text("Saved")
+                        .foregroundStyle(.green)
+                        .font(.callout)
+                }
+                Spacer()
+                Button("Cancel") { dismiss() }
+                    .keyboardShortcut(.cancelAction)
+                Button("Save") {
+                    if model.save() { dismiss() }
+                }
+                .keyboardShortcut(.defaultAction)
+            }
+            .padding(12)
+        }
+        .frame(width: 560, height: 620)
+        .background(Theme.background)
+    }
+}

Sources/ICCery/SettingsViewModel.swift

@@ -0,0 +1,68 @@
+import AppKit
+import Foundation
+import ICCeryCore
+
+/// Backs the Settings sheet (issue #5). Load → edit → save with
+/// validation; the log level is applied live via `LogSink` (#158) and a
+/// `settingsDidChange` notification fans out to #20.
+@MainActor
+@Observable
+final class SettingsViewModel {
+
+    var settings: AppSettings
+    var validationErrors: [String] = []
+    var savedFlash = false
+
+    private let store: SettingsStore
+    private let sink: LogSink
+
+    init(store: SettingsStore = SettingsStore(), sink: LogSink = .shared) {
+        self.store = store
+        self.sink = sink
+        self.settings = store.load()
+    }
+
+    /// Persists after validation. Returns false (and shows inline
+    /// errors) when the form is invalid.
+    @discardableResult
+    func save() -> Bool {
+        validationErrors = settings.validate()
+        guard validationErrors.isEmpty else { return false }
+        do {
+            try store.save(settings)
+            sink.applySettings(settings)
+            savedFlash = true
+            Task {
+                try? await Task.sleep(for: .seconds(1.5))
+                savedFlash = false
+            }
+            return true
+        } catch {
+            validationErrors = ["Could not save settings: \(error.localizedDescription)"]
+            return false
+        }
+    }
+
+    // MARK: - Log helpers
+
+    var logFileURL: URL { AppPaths.logFile }
+
+    func openLogFolder() {
+        try? FileManager.default.createDirectory(
+            at: AppPaths.logDir, withIntermediateDirectories: true
+        )
+        NSWorkspace.shared.selectFile(
+            AppPaths.logFile.path, inFileViewerRootedAtPath: AppPaths.logDir.path
+        )
+    }
+
+    func copyLogPath() {
+        NSPasteboard.general.clearContents()
+        NSPasteboard.general.setString(AppPaths.logFile.path, forType: .string)
+    }
+
+    func copyLogExcerpt() {
+        NSPasteboard.general.clearContents()
+        NSPasteboard.general.setString(sink.tailExcerpt(), forType: .string)
+    }
+}

Tests/ICCeryCoreTests/SettingsTests.swift

@@ -0,0 +1,164 @@
+import Testing
+import Foundation
+@testable import ICCeryCore
+
+private func tempStoreURL() -> URL {
+    FileManager.default.temporaryDirectory
+        .appendingPathComponent("iccery-settings-\(UUID().uuidString)")
+        .appendingPathComponent("settings.json")
+}
+
+@Suite("AppSettings")
+struct AppSettingsTests {
+    @Test func defaults() {
+        let s = AppSettings.default
+        #expect(s.argyllBinaryDir == nil)
+        #expect(s.defaultInstrument == nil)
+        #expect(s.logLevel == nil)
+        #expect(s.deltaEGoodMax == 2.0)
+        #expect(s.deltaEWarningMax == 5.0)
+        #expect(s.customPresets.isEmpty)
+        #expect(!s.enableI1Pro2Leds)
+        #expect(s.calibrationStaleDays == 30)
+        #expect(s.defaultInstallLocation == .user)
+        #expect(s.askBeforeOverwriteProfile)
+        #expect(!s.openColorPanelAfterInstall)
+        #expect(s.isValid)
+    }
+
+    @Test func negativeThresholds() {
+        var s = AppSettings.default
+        s.deltaEGoodMax = -1
+        #expect(s.validate() == [AppSettings.errorNegativeDeltaE])
+        s.deltaEGoodMax = 2.0
+        s.deltaEWarningMax = -0.5
+        // -0.5 < 0 → negative error; good(2.0) >= warn(-0.5) → order error too
+        #expect(s.validate() == [
+            AppSettings.errorNegativeDeltaE,
+            AppSettings.errorThresholdOrder,
+        ])
+    }
+
+    @Test func goodMustBeStrictlyLessThanWarning() {
+        var s = AppSettings.default
+        s.deltaEGoodMax = 5.0
+        #expect(s.validate() == [AppSettings.errorThresholdOrder])
+        s.deltaEGoodMax = 6.0
+        #expect(s.validate() == [AppSettings.errorThresholdOrder])
+        s.deltaEGoodMax = 4.9
+        #expect(s.isValid)
+    }
+
+    @Test func snakeCaseKeys() throws {
+        let s = AppSettings.default
+        let data = try JSONEncoder().encode(s)
+        let json = String(data: data, encoding: .utf8)!
+        #expect(json.contains("\"delta_e_good_max\""))
+        #expect(json.contains("\"default_install_location\""))
+        #expect(json.contains("\"enable_i1pro2_leds\""))
+    }
+}
+
+@Suite("SettingsStore")
+struct SettingsStoreTests {
+    @Test func roundTrip() throws {
+        let url = tempStoreURL()
+        let store = SettingsStore(fileURL: url)
+        var s = AppSettings.default
+        s.deltaEGoodMax = 1.5
+        s.defaultInstrument = "p3"
+        try store.save(s)
+        #expect(store.load() == s)
+    }
+
+    @Test func corruptJsonFallsBackToDefaults() throws {
+        let url = tempStoreURL()
+        try FileManager.default.createDirectory(
+            at: url.deletingLastPathComponent(), withIntermediateDirectories: true
+        )
+        try "{ not json".write(to: url, atomically: true, encoding: .utf8)
+        #expect(SettingsStore(fileURL: url).load() == .default)
+    }
+
+    @Test func missingFileReturnsDefaults() {
+        #expect(SettingsStore(fileURL: tempStoreURL()).load() == .default)
+    }
+
+    @Test func invalidSettingsNotPersisted() throws {
+        let url = tempStoreURL()
+        let store = SettingsStore(fileURL: url)
+        var s = AppSettings.default
+        s.deltaEGoodMax = 9.0 // >= warning 5.0
+        #expect(throws: SettingsStore.SettingsError.self) { try store.save(s) }
+        #expect(!FileManager.default.fileExists(atPath: url.path))
+    }
+
+    @Test func savePostsNotification() async throws {
+        let url = tempStoreURL()
+        let store = SettingsStore(fileURL: url)
+        var fired = false
+        let token = NotificationCenter.default.addObserver(
+            forName: SettingsStore.settingsDidChange, object: nil, queue: nil
+        ) { _ in fired = true }
+        defer { NotificationCenter.default.removeObserver(token) }
+        try store.save(.default)
+        #expect(fired)
+    }
+}
+
+@Suite("LogSink")
+struct LogSinkTests {
+    private func tempLog() -> (URL, LogSink) {
+        let url = FileManager.default.temporaryDirectory
+            .appendingPathComponent("iccery-log-\(UUID().uuidString)")
+            .appendingPathComponent("iccery.log")
+        return (url, LogSink(fileURL: url))
+    }
+
+    @Test func writesFormattedLines() {
+        let (url, sink) = tempLog()
+        sink.setLevel(.debug)
+        sink.write(level: .info, category: "test", message: "hello")
+        let content = (try? String(contentsOf: url, encoding: .utf8)) ?? ""
+        #expect(content.contains("[INFO] test: hello"))
+    }
+
+    @Test func levelFilteringIsLive() {
+        let (url, sink) = tempLog()
+        sink.setLevel(.error)
+        sink.write(level: .info, category: "t", message: "hidden")
+        sink.setLevel(.info)   // runtime change, no restart (#158)
+        sink.write(level: .info, category: "t", message: "shown")
+        let content = (try? String(contentsOf: url, encoding: .utf8)) ?? ""
+        #expect(!content.contains("hidden"))
+        #expect(content.contains("shown"))
+    }
+
+    @Test func rotatesAt5MiBKeeping5Segments() throws {
+        let (url, sink) = tempLog()
+        sink.setLevel(.trace)
+        // Pre-fill the active log just under the cap, then cross it.
+        try FileManager.default.createDirectory(
+            at: url.deletingLastPathComponent(), withIntermediateDirectories: true
+        )
+        let big = String(repeating: "x", count: Int(LogSink.maxSegmentBytes))
+        try big.write(to: url, atomically: true, encoding: .utf8)
+
+        sink.write(level: .info, category: "t", message: "trigger rotation")
+        #expect(FileManager.default.fileExists(
+            atPath: url.appendingPathExtension("1").path
+        ))
+        // Active log is small again.
+        let size = try FileManager.default.attributesOfItem(
+            atPath: url.path
+        )[.size] as? UInt64
+        #expect((size ?? 0) < 1024)
+    }
+
+    @Test func tailExcerptCaps() throws {
+        let (url, sink) = tempLog()
+        sink.setLevel(.debug)
+        sink.write(level: .info, category: "t", message: "line")
+        #expect(sink.tailExcerpt(maxBytes: 8).count <= 8)
+    }
+}