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/src/player/android/mod.rs

@@ -420,7 +420,18 @@ impl PlayerBackend for ExoPlayerBackend {
             None => JValue::Object(&null_obj),
         };
 
-        // Determine media type string for JNI
+        // Determine media type string for JNI.
+        //
+        // This is not cosmetic: the string decides *which audio-focus mechanism*
+        // runs on the Kotlin side. `JellyTauPlayer.load()` re-applies
+        // `setAudioAttributes(attrs, handleAudioFocus = mediaType == AUDIO)`, so
+        // "audio" leaves focus to ExoPlayer (request on play, duck on transient
+        // loss, pause on a call) while "video" switches it to the manual
+        // `AudioFocusRequest` path, which needs delayed-focus handling. Either
+        // way the resulting pause comes back through `nativeOnStateChanged`, so
+        // the Rust controller — not the focus listener — stays authoritative.
+        //
+        // TRACES: UR-004, UR-006 | IR-008
         let media_type_str = match media.media_type {
             MediaType::Video => "video",
             MediaType::Audio => "audio",
@@ -1096,6 +1107,15 @@ pub extern "system" fn Java_com_dtourolle_jellytau_player_JellyTauPlayer_nativeO
 ///
 /// Commands from lockscreen controls, notification buttons, and Bluetooth
 /// devices are routed through here to the Rust PlayerController.
+///
+/// This is the inbound half of UR-006: `MediaSessionCompat` is flagged
+/// `FLAG_HANDLES_MEDIA_BUTTONS`, so an AVRCP play/pause/skip from a headset
+/// arrives at the service's transport callback and lands here as a command
+/// string. The player stays authoritative — the session is a consumer that
+/// *requests*, and the resulting state comes back out through
+/// [`update_lockscreen_metadata`].
+///
+/// TRACES: UR-006 | IR-006
 #[no_mangle]
 pub extern "system" fn Java_com_dtourolle_jellytau_player_JellyTauPlaybackService_nativeOnMediaCommand(
     mut env: JNIEnv,
@@ -1396,6 +1416,8 @@ use crate::player::LockscreenMetadata;
 /// running (in remote mode it is started via [`enable_remote_volume`]); if it
 /// isn't, this is a no-op rather than an error so it can be called freely on
 /// every poll tick.
+///
+/// TRACES: UR-006 | IR-006
 pub fn update_lockscreen_metadata(meta: &LockscreenMetadata) -> Result<(), String> {
     let vm = JAVA_VM.get().ok_or("JavaVM not initialized")?;
     let mut env = vm.attach_current_thread().map_err(|e| e.to_string())?;