feat(16-03): WriteTrackTags pipeline with player safety, scan mutex, events, and app wiring
- TagWriter struct with WriteTrackTags entry point orchestrating full pipeline
- PlayerStopper interface + playerAdapter for import-cycle-free player safety
- PipelineLocker interface + pipelineMu on Library for scan/write mutual exclusion
- TrackMetadataChanged event constant + auto-generated TypeScript events
- TagWriter wired into app.go as Wails binding with SetContext in OnStartup
- 5 pipeline integration tests: player safety, scan mutex, orphan cleanup, genre relink, full DB sync
- Lint fixes: static error, else-if, wsl cuddle, golines