docs(traces): tag the twelve "Done but untraced" requirements, and stop the matrix over-reporting

Twelve requirements were marked Done in docs/requirements.md with zero TRACES
anywhere in the tree. The features work — the tags were simply never written —
so the matrix over-reported on exactly the requirements a reviewer would most
want to verify. Each is now tagged at the code that actually implements it:

- JA-006 / JA-009 / JA-013 / JA-014 / JA-015 / JA-018 and IR-022 / IR-024 at
  their Jellyfin call sites in repository/online.rs (search, get_item's
  MediaStreams/People fields, Items/Resume, Shows/NextUp, FavoriteItems DELETE,
  get_person/get_items_by_person), plus the commands that expose them.
- UR-006 / IR-006 across the lockscreen spine: JellyTauPlaybackService (the
  MediaSessionCompat owner), the nativeOnMediaCommand JNI intake, and
  LockscreenMetadata / update_lockscreen_metadata.
- IR-008 at both audio-focus mechanisms — ExoPlayer-managed for audio, the
  manual AudioFocusRequest listener for video — and at the media-type string
  that chooses between them.
- UR-037 (with DR-042, also untraced) on the video-library poster grid:
  LibraryGrid, MediaCard, and the tv/movies routes.

Resolve contradictory statuses across layers, evidence first:

- IR-018/IR-019 were Planned under Done URs because they were scoped to libmpv.
  MpvBackend is the audio-only backend and overrides neither
  set_subtitle_track nor set_audio_track — the trait's not_implemented()
  default still stands — so UR-020/UR-021 are met by ExoPlayer and by the
  HTML5 <video> path instead. Both IRs are re-scoped to those backends and
  marked Done; IT-008/IT-009 and the stale @req-planned markers in backend.rs
  follow.
- IR-005 (MPRIS) stays Planned: there is no MPRIS/D-Bus code or dependency in
  the project and update_lockscreen_metadata is a no-op off Android. UR-006 is
  corrected to Done (Android) rather than the IR being marked Done.
- A note under the IR table records where a UR is met by a different mechanism
  than its IR anticipated.

Define the two dangling IDs the source already referenced: DR-189 (the control
bar never auto-hid on a touchscreen, because its timer was armed only from
onmousemove) and UT-188 (its rule test). The live-denominator assertion in
extract-traces.test.ts moves 187/330 to 188/331 accordingly.

Traced requirements 444 to 459; IR coverage 19/32 to 25/32.

src-tauri/android/src/main/java/com/dtourolle/jellytau/player/JellyTauPlaybackService.kt

@@ -27,6 +27,17 @@ import com.google.common.util.concurrent.ListenableFuture
  *
  * Media commands are routed back to Rust via JNI to ensure proper
  * queue management for next/previous track operations.
+ *
+ * This class owns both sessions: the media3 [MediaSession] the service contract
+ * requires, and the legacy [MediaSessionCompat] that actually carries the
+ * lockscreen transport. The compat session is flagged
+ * FLAG_HANDLES_MEDIA_BUTTONS or FLAG_HANDLES_TRANSPORT_CONTROLS, which is what
+ * makes a Bluetooth headset's AVRCP play/pause/skip arrive as a transport
+ * callback; every one of those callbacks is forwarded to Rust through
+ * nativeOnMediaCommand rather than acted on locally, so the player stays the
+ * single source of truth and the session remains a consumer of its state.
+ *
+ * TRACES: UR-006 | IR-006
  */
 @OptIn(UnstableApi::class)
 class JellyTauPlaybackService : MediaSessionService() {

src-tauri/android/src/main/java/com/dtourolle/jellytau/player/JellyTauPlayer.kt

@@ -263,7 +263,15 @@ class JellyTauPlayer(private val appContext: Context) {
             .setContentType(C.AUDIO_CONTENT_TYPE_MUSIC)
             .build()
 
-        // Create ExoPlayer with audio focus handling
+        // Create ExoPlayer with audio focus handling.
+        //
+        // For audio playback ExoPlayer manages focus itself: handleAudioFocus=true
+        // makes it request AUDIOFOCUS_GAIN on play, duck on a transient loss, and
+        // pause on a call or another app taking focus. Video re-applies this per
+        // load with handleAudioFocus=false and drives focus manually instead (see
+        // requestAudioFocus), because a video needs delayed-focus handling.
+        //
+        // TRACES: UR-004, UR-006 | IR-008
         exoPlayer = ExoPlayer.Builder(appContext)
             .setAudioAttributes(audioAttributes, /* handleAudioFocus= */ true)
             // Pause when the audio output is removed (wired headphones unplugged or
@@ -1328,7 +1336,12 @@ class JellyTauPlayer(private val appContext: Context) {
      * Request audio focus for video playback.
      * This is critical for video to have audio on Android.
      *
-     * TRACES: UR-004 | DR-145
+     * The listener installed here is the pause-on-call path: AUDIOFOCUS_LOSS and
+     * AUDIOFOCUS_LOSS_TRANSIENT (an incoming call is the latter) both pause,
+     * LOSS_TRANSIENT_CAN_DUCK lowers the volume instead, and GAIN restores —
+     * resuming only what we paused, via pendingPlayOnFocusGain.
+     *
+     * TRACES: UR-004, UR-006 | IR-008, DR-145
      *
      * @return true if focus was granted outright and playback may start now.
      *   false for a DELAYED or refused request — the caller must hold playback