feat: native Android audio via Media3 ExoPlayer + full NSBridge JS adapter

- Replace legacy MediaBrowserServiceCompat/MediaSessionCompat with
  Media3 MediaSessionService (MusicService.kt) + ExoPlayer set to
  WAKE_MODE_NETWORK — holds WakeLock + WifiLock through Doze so audio
  plays uninterrupted on locked screen
- Media3 manages the notification; no manual NotificationCompat.Builder
  → no disappear/reappear flicker when skipping tracks
- Full bidirectional bridge:
    JS → native: window.NSBridge.* (WebBridge.kt, 13 methods)
    Native → JS: window.NSPlayer.* callbacks (onReady, onTrackChanged,
      onPositionUpdate, onPlaybackStateChanged, onQueueEnded, onError)
- player.js: progressive enhancement — all playback routes through
  NSBridge when running inside the wrapper, falls back to S.audio on
  plain PWA/browser; removes all window.Android.* calls
- main.js: guard _onForeground mediaSession playbackState setter behind
  !window.NSBridge so native Media3 session is never stomped
- ProGuard rules updated to keep WebBridge @JavascriptInterface methods
  and Media3 classes
- bridge-test.html: manual integration test page for every bridge method

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/main.js

@@ -166,10 +166,12 @@ window.addEventListener('load', () => {
     const s = S.queue[S.qIdx];
     if(!s) return;
     updateMediaSession(s);
-    // Don't stomp playbackState during a src-swap — _trackChanging guards the transition
-    if(navigator.mediaSession && !S._trackChanging)
+    // Don't stomp playbackState during a src-swap — _trackChanging guards the transition.
+    // Skip when native is active: Media3 owns the system media session.
+    if(navigator.mediaSession && !S._trackChanging && !window.NSBridge)
       navigator.mediaSession.playbackState = S.audio.paused ? 'paused' : 'playing';
-    // If audio was playing before backgrounding but is now paused (OS kill), try to resume
+    // If audio was playing before backgrounding but is now paused (OS kill), try to resume.
+    // No-op when native (S.audio.src is empty; native handles its own recovery).
     if(!S.audio.paused) return;
     if(s && S.audio.src) S.audio.play().catch(() => { S._trackChanging = false; });
   }