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.

docs/specs/audio-equalizer.md

@@ -4,6 +4,7 @@
 **Requirements:** UR-027 → DR-030 (EQ UI), IR-020 (MPV EQ integration).
 **UX spec:** n/a (extends the Settings › Audio section, ux-flows §8.1 instant-apply).
 **Supersedes / revises:** —
+**Revised by:** [android-audio-settings-parity.md](android-audio-settings-parity.md) — lifts the "Android is a no-op" limitation below.
 
 ## Summary
 
@@ -115,6 +116,10 @@ fields ride along. `NullBackend`/Android inherit the trait default (no-op).
 ## Out of scope
 
 - Android/ExoPlayer EQ (parity gap tracked with crossfade/gapless/normalize).
+  **Now specified in [android-audio-settings-parity.md](android-audio-settings-parity.md)**,
+  which implements `set_audio_settings` on `ExoPlayerBackend`. The canonical band
+  layout and preset→curve map defined here remain authoritative; the Android side
+  resamples those bands onto the device equalizer rather than defining its own.
 - Per-track or per-library EQ profiles — one global profile only.
 - Automatic loudness/room correction; only manual bands + presets.
 - Changing the crossfade/normalize TODOs in `set_audio_settings` beyond wiring