feat/5-settings-logging
- AppSettings: snake_case Codable model — argyll_binary_dir, default_instrument (stored, never applied to argv), log_level (nil -> Debug debug / Info release), delta_e thresholds (2.0/5.0), custom_presets, enable_i1pro2_leds, calibration_stale_days 30, default_install_location user, ask_before_overwrite_profile, open_color_panel_after_install - Validation with the exact contract strings; save() refuses invalid settings; corrupt/missing JSON -> defaults; settingsDidChange notification posted on save (for #20) - LogSink: rolling file at ~/Library/Logs/com.gronod.iccery2/ iccery.log, 5 MiB x 5 segments, runtime setLevel applied at startup and on save (#158); AppLogger gates os_log+file through it - SettingsView sheet: Argyll dir picker, instrument (display-only caveat), i1Pro2 LEDs, ΔE fields + inline errors, stale days, install location, overwrite + ColorSync toggles, log level, open-log-folder / copy-path / copy-excerpt - v1 settings path never read; writes atomic via AtomicFileWriter - 13 new tests; 60/60 green Generated with [Devin](https://devin.ai) Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>