fix(player): make lockscreen transport reach background audio (DR-097)

Pausing from the lockscreen did nothing while a video's audio played in
the background. The handoff starts native ExoPlayer audio and only then
tears the WebView <video> down, and that teardown fires a DOM `pause`
the frontend reports like any other — leaving html5_playing = Some(false).
Transport therefore stayed aimed at the element: the lockscreen pause
emitted a ControlCommand into a <video> that no longer existed while the
native player carried on.

The controller now tracks a background-audio handoff explicitly. Entering
one hands transport authority to the native backend and drops the dying
element's state/position/media-loaded reports, which also stop flipping
the UI to paused and dragging the position backwards. Exiting restores
the element as the player.

A lockscreen pause also has to survive the return to the foreground: the
video used to resume from a snapshot taken at handoff time, undoing the
pause on the way back in. shouldResumeOnForeground() lets an explicit
`paused` from the player override that snapshot.

TRACES: UR-040, UR-005 | DR-052, DR-097

src-tauri/src/player/mod.rs

@@ -179,6 +179,17 @@ pub struct PlayerController {
     // TRACES: UR-040 | DR-052
     background_audio_base: Arc<Mutex<f64>>,
 
+    // True while a background-audio handoff owns playback: the native audio
+    // player is the real player and the webview <video> has been torn down.
+    //
+    // The teardown is what makes this necessary. It fires a DOM `pause` that the
+    // frontend reports like any other, which would otherwise leave the controller
+    // believing webview media is still active — aiming lockscreen transport at an
+    // element that no longer exists (see `is_html5_active`).
+    //
+    // TRACES: UR-040 | DR-052, DR-097
+    background_audio_active: Arc<Mutex<bool>>,
+
     // Budget for re-opening a stream that ended short of the item's runtime.
     //
     // A resume re-requests the same URL, so a server that is genuinely gone would
@@ -221,6 +232,7 @@ impl PlayerController {
             end_reason: Arc::new(Mutex::new(None)),
             autoplay_episode_count: Arc::new(Mutex::new(0)),
             background_audio_base: Arc::new(Mutex::new(0.0)),
+            background_audio_active: Arc::new(Mutex::new(false)),
             stream_resume: Arc::new(Mutex::new(stream_end::ResumeTracker::default())),
             html5_playing: Arc::new(Mutex::new(None)),
         };
@@ -1000,6 +1012,14 @@ impl PlayerController {
     /// Re-emits a `StateChanged` event identical to what MpvBackend/ExoPlayer
     /// would emit, so `playerEvents.ts` needs no HTML5-specific branch.
     pub fn report_html5_state(&self, state: String, media_id: Option<String>) {
+        // A background-audio handoff has already moved playback to the native
+        // player and torn the element down; anything it still reports describes
+        // a video that is no longer playing. Dropping it keeps the UI on the
+        // audio that IS playing and leaves transport with the native backend.
+        if self.is_background_audio_active() {
+            debug!("[PlayerController] Ignoring HTML5 state '{state}' during background audio");
+            return;
+        }
         // Track it: this is the authoritative play/pause state for
         // webview-rendered media, and what transport decisions read (DR-097).
         // "stopped"/"idle" mean the element is gone, so hand authority back to
@@ -1027,6 +1047,11 @@ impl PlayerController {
     /// Re-emits a `PositionUpdate` event mirroring the native backends' periodic
     /// position updates (the adapter is expected to throttle to ~250ms like MPV).
     pub fn report_html5_position(&self, position: f64, duration: f64) {
+        // Stale by definition during a handoff — the native player's ticks are
+        // the real position. See `report_html5_state`.
+        if self.is_background_audio_active() {
+            return;
+        }
         if let Some(emitter) = self.event_emitter.lock_safe().as_ref() {
             emitter.emit(PlayerStatusEvent::PositionUpdate { position, duration });
         }
@@ -1035,6 +1060,10 @@ impl PlayerController {
     /// Report that the HTML5 <video> element finished loading and knows its
     /// duration. Mirrors the native `MediaLoaded` event.
     pub fn report_html5_media_loaded(&self, duration: f64) {
+        // See `report_html5_state` — the element is not the player right now.
+        if self.is_background_audio_active() {
+            return;
+        }
         if let Some(emitter) = self.event_emitter.lock_safe().as_ref() {
             emitter.emit(PlayerStatusEvent::MediaLoaded { duration });
         }
@@ -1234,6 +1263,42 @@ impl PlayerController {
         *self.background_audio_base.lock_safe() = seconds.max(0.0);
     }
 
+    /// Enter a background-audio handoff at `position` (the video's position, and
+    /// therefore the audio stream's zero).
+    ///
+    /// Hands transport authority to the native audio player: the webview
+    /// `<video>` is about to be torn down, so its last reports — including the
+    /// `pause` the teardown itself fires — must not keep it looking like the
+    /// player. Without this the lockscreen pause emitted a ControlCommand at a
+    /// dead element and the audio played straight through it.
+    ///
+    /// TRACES: UR-040, UR-005 | DR-052, DR-097
+    pub fn enter_background_audio(&self, position: f64) {
+        self.set_background_audio_base(position);
+        *self.background_audio_active.lock_safe() = true;
+        *self.html5_playing.lock_safe() = None;
+    }
+
+    /// Leave a background-audio handoff, returning the base offset to add to the
+    /// native player's relative position.
+    ///
+    /// The webview `<video>` becomes the player again once it reloads, so its
+    /// reports are honoured from here on.
+    ///
+    /// TRACES: UR-040, UR-005 | DR-052, DR-097
+    pub fn exit_background_audio(&self) -> f64 {
+        *self.background_audio_active.lock_safe() = false;
+        self.take_background_audio_base()
+    }
+
+    /// True while the native audio player owns playback via a background-audio
+    /// handoff.
+    ///
+    /// TRACES: UR-040 | DR-052
+    pub fn is_background_audio_active(&self) -> bool {
+        *self.background_audio_active.lock_safe()
+    }
+
     /// Read and clear the background-audio base offset.
     ///
     /// TRACES: UR-040 | DR-052
@@ -2002,6 +2067,84 @@ mod tests {
         assert_eq!(controls, vec!["play".to_string(), "pause".to_string()]);
     }
 
+    #[test]
+    fn test_background_audio_handoff_moves_transport_to_native_backend() {
+        // Lockscreen pause while playing a video's audio in the background.
+        //
+        // The handoff tears the WebView <video> down AFTER native audio starts,
+        // and that teardown fires a DOM `pause` the frontend dutifully reports.
+        // That report used to leave `html5_playing = Some(false)`, so transport
+        // kept being aimed at an element that no longer exists: the lockscreen
+        // pause emitted a ControlCommand into the void and the audio played on.
+        let controller = PlayerController::default();
+        let emitter = Arc::new(CapturingEmitter::new());
+        controller.set_event_emitter(emitter.clone());
+
+        // Video was playing in the webview.
+        controller.report_html5_state("playing".to_string(), Some("ep-1".to_string()));
+        assert!(controller.is_html5_active());
+
+        // Hand off to the native audio player, then tear the element down.
+        controller.enter_background_audio(1200.0);
+        controller.report_html5_state("paused".to_string(), Some("ep-1".to_string()));
+
+        assert!(
+            !controller.is_html5_active(),
+            "native audio owns transport during a background-audio handoff"
+        );
+
+        controller.pause().unwrap();
+        let controls: Vec<_> = emitter
+            .events()
+            .into_iter()
+            .filter_map(|e| match e {
+                PlayerStatusEvent::ControlCommand { action, .. } => Some(action),
+                _ => None,
+            })
+            .collect();
+        assert!(
+            controls.is_empty(),
+            "pause must drive the native backend, not a torn-down element: {:?}",
+            controls
+        );
+    }
+
+    #[test]
+    fn test_background_audio_handoff_suppresses_stale_element_events() {
+        // The dying element's pause/position reports describe the video, not the
+        // audio now playing — re-emitting them flips the UI to paused and yanks
+        // the position backwards while native audio keeps going.
+        let controller = PlayerController::default();
+        let emitter = Arc::new(CapturingEmitter::new());
+        controller.set_event_emitter(emitter.clone());
+
+        controller.enter_background_audio(1200.0);
+        controller.report_html5_state("paused".to_string(), Some("ep-1".to_string()));
+        controller.report_html5_position(1200.0, 2400.0);
+
+        assert!(
+            emitter.events().is_empty(),
+            "stale webview reports must not reach the event pipeline: {:?}",
+            emitter.events()
+        );
+    }
+
+    #[test]
+    fn test_exit_background_audio_returns_transport_to_the_webview() {
+        // Back in the foreground the <video> is the player again, so its reports
+        // must be honoured — and the base offset still comes back for the resume.
+        let controller = PlayerController::default();
+        let emitter = Arc::new(CapturingEmitter::new());
+        controller.set_event_emitter(emitter.clone());
+
+        controller.enter_background_audio(1200.0);
+        assert_eq!(controller.exit_background_audio(), 1200.0);
+
+        controller.report_html5_state("playing".to_string(), Some("ep-1".to_string()));
+        assert!(controller.is_html5_active());
+        assert!(controller.html5_is_playing());
+    }
+
     #[test]
     fn test_html5_stopped_report_releases_transport_to_native_backend() {
         // When webview video goes away, transport must fall back to the native