docs(player): backend unification findings + correct false parity claims

Investigation into unifying the playback backends (Linux/MPV, Android/ExoPlayer,
Windows/webview) onto one engine with hardware acceleration. Conclusion: video
cannot be unified onto a native engine; audio can.

The blocker is not mpv-specific. WebKitGTK, WebView2 and Android WebView each
draw into their own compositor surface, so a native video surface sits either
entirely above or entirely below the webview and cannot interleave with HTML.
GStreamer and libVLC fail identically. mpv would additionally regress streaming:
it has no adaptive bitrate, while the current hls.js path does.

Six specs added:
- playback-backend-unification: the analysis and decision, with evidence
- android-audio-settings-parity: set_audio_settings on ExoPlayerBackend
- android-native-video-spike: timeboxed test of SurfaceView compositing
- windows-native-audio-backend: replace the webview <audio> shim with libmpv
- libmpv2-migration: dead libmpv git pin -> libmpv2, plus a LICENSE file
- playback-docs-corrections: the requirement-status fixes applied here

Corrections to requirements.md, all verified against source:
- UR-031/DR-034 claimed crossfade was "Done (Linux only)". It is implemented
  nowhere (mpv_backend.rs has a bare TODO) and is architecturally blocked on
  mpv, whose single-stream audio chain cannot feed acrossfade's two inputs.
- Parity matrix listed crossfade as a Linux/Android gap; it is neither.
- The matrix omitted the equalizer, which has the same Linux-only shape.
- The suggested ConcatenatingMediaSource is deprecated in current Media3.

nativeAdapter.ts cited tauri#10152 as an upstream blocker for native Android
video. That issue is a stale feature request, dead since 2024-07-01; the
capability shipped in tauri 27d01834 (2024-09-02), and the related
black-screen bug was fixed in wry 0.39.4 (we ship 0.55.x). What is genuinely
unproven is SurfaceView-behind-WebView compositing, which the spike now tracks.

src/lib/player/adapters/nativeAdapter.ts

@@ -7,11 +7,19 @@
  * hls.js. State reporting is unnecessary here because the native backend emits
  * events directly — the adapter's job is only to forward control intents.
  *
- * NOTE: On current Tauri, native Android video rendering is blocked upstream
- * (transparent webview / SurfaceView compositing — tauri#10152), so video on
- * Android currently runs through the HTML5 adapter via the interim override in
- * the factory. This adapter exists for the audio/native path and for when that
- * upstream limitation is resolved.
+ * NOTE: This adapter is currently unreachable — `createAdapter()` hardcodes the
+ * HTML5 kind, so Android video runs through Html5PlayerAdapter.
+ *
+ * That override was introduced citing tauri#10152 as an upstream blocker. That
+ * is no longer accurate: #10152 is a stale *feature request* (dead since
+ * 2024-07-01) asking that `transparent` not be desktop-only, and the capability
+ * shipped in tauri commit 27d01834 (2024-09-02). The related black/white-screen
+ * bug (tauri#8381, #9408) was a broken JNI signature for setBackgroundColor,
+ * fixed in wry 0.39.4; we ship wry 0.55.x.
+ *
+ * What is genuinely unproven is SurfaceView-behind-WebView *compositing* on
+ * Tauri Android — nothing upstream blocks it, and nothing upstream demonstrates
+ * it either. docs/specs/android-native-video-spike.md tracks that experiment.
  *
  * TRACES: UR-003, UR-005 | DR-004, DR-028
  */