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

frontend/src/events.ts

@@ -47,6 +47,9 @@ export const Events = {
     LibraryAdded: "LibraryAdded",
     LibraryRenamed: "LibraryRenamed",
     LibraryRemoved: "LibraryRemoved",
+
+    // Tag writing events
+    TrackMetadataChanged: "TrackMetadataChanged",
 } as const;
 
 export type EventName = (typeof Events)[keyof typeof Events];