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/commands/player/mod.rs

@@ -1478,6 +1478,16 @@ pub async fn player_seek_video(
 
 /// Switch audio track - handles both HTML5 (stream reload) and native (direct switch)
 /// Note: Frontend should handle saving series preferences after this command succeeds
+///
+/// The split is the requirement: an HTML5 `<video>` element cannot be told to
+/// change audio track, so the stream is re-opened at the chosen
+/// `AudioStreamIndex` and the frontend seeks the reloaded element back to
+/// `position`; a native backend (ExoPlayer) switches in place by track-group
+/// index. libmpv implements neither — it is the audio-only backend here and
+/// leaves `PlayerBackend::set_audio_track` at its `not_implemented()` default,
+/// which is why IR-019 is met by these two paths rather than by MPV.
+///
+/// TRACES: UR-021 | IR-019, DR-024
 #[tauri::command]
 #[specta::specta]
 pub async fn player_switch_audio_track(
@@ -1650,6 +1660,9 @@ pub async fn player_set_stream_quality(
     Ok(StreamQualityResponse::Native { position })
 }
 
+/// Set the active audio track on a native backend directly.
+///
+/// TRACES: UR-021 | IR-019, DR-024
 #[tauri::command]
 #[specta::specta]
 pub async fn player_set_audio_track(
@@ -1663,6 +1676,14 @@ pub async fn player_set_audio_track(
     Ok(get_player_status(&controller))
 }
 
+/// Set (or clear, with `None`) the active subtitle track on a native backend.
+///
+/// On Android this indexes ExoPlayer's *text track groups* — i.e. the position
+/// of the sideloaded `MediaItem.SubtitleConfiguration`, not the Jellyfin stream
+/// index. The HTML5 path never reaches here; it toggles its own `<track>`
+/// children. libmpv implements neither, leaving the trait default in place.
+///
+/// TRACES: UR-020 | IR-018, DR-023
 #[tauri::command]
 #[specta::specta]
 pub async fn player_set_subtitle_track(

src-tauri/src/commands/repository.rs

@@ -1,7 +1,7 @@
 //! Tauri commands for repository access
 //! Uses handle-based system: UUID -> Arc<HybridRepository>
 //!
-//! TRACES: UR-007, UR-035, UR-036 | JA-004, JA-005, JA-029, JA-030, JA-031
+//! TRACES: UR-007, UR-008, UR-023, UR-034, UR-035, UR-036 | IR-022, IR-024, JA-004, JA-005, JA-006, JA-029, JA-030, JA-031
 
 use crate::utils::lock::MutexSafe;
 use std::collections::HashMap;
@@ -294,7 +294,13 @@ pub async fn repository_get_latest_items(
         .map_err(|e| format!("{:?}", e))
 }
 
-/// Get resume items (continue watching/listening)
+/// Get resume items (continue watching/listening).
+///
+/// The home screen's Continue Watching row and every library's "pick up where
+/// you left off" hero come through here; each item carries its own resume
+/// position in `UserData`.
+///
+/// TRACES: UR-019, UR-023, UR-034 | IR-024, JA-013, JA-015 | DR-026, DR-038
 #[tauri::command]
 #[specta::specta]
 pub async fn repository_get_resume_items(
@@ -318,7 +324,9 @@ pub async fn repository_get_resume_items(
         })
 }
 
-/// Get next up episodes
+/// Get next up episodes.
+///
+/// TRACES: UR-023, UR-034 | IR-024, JA-014 | DR-026
 #[tauri::command]
 #[specta::specta]
 pub async fn repository_get_next_up_episodes(