Adds TagWriter protocol, TagWriterFactory, MP4TagWriter (AVFoundation
passthrough export) and ID3TagWriter (ID3TagEditor 5.5.0 v2.3 builder),
with round-trip tests for both formats and fixture audio files.
Add chase-seek scrubbing for smooth slider interaction, global keyboard
monitor for space/arrows, now-playing speaker column in track table, Home
button in playlist bar, smart playlist sorting, and UI polish (label colors,
track selection, app icon 512@2x). Bump version to 10.
Also fixes unit test infrastructure: create shared scheme including
MusicTests, fix TEST_HOST path (Mumu.app), add PRODUCT_MODULE_NAME=Music,
add get-task-allow entitlement, and skip NSOpenPanel in test runs.
Introduces the core Track struct (nonisolated to work with project-wide
MainActor default isolation), its FetchableRecord/MutablePersistableRecord
conformances, a DEBUG fixture helper, and three Swift Testing tests covering
round-trip persistence, ID assignment on insert, and deterministic hash computation.
Laurent