fix(player): lockscreen skip scrubs instead of advancing in background audio

onSkipToNext/onSkipToPrevious forwarded a bare next/previous to Rust, which
always advanced the queue. Correct for music, wrong for a video whose audio is
running through a background-audio handoff (UR-040): pressing skip to re-hear a
line jumped to the next episode instead of scrubbing.

resolve_skip_action in player/seek.rs maps the command to Advance or SeekTo, and
is_background_audio_active() is the whole test — the handoff exists only for
video, and an episode played through it reports MediaType::Audio, so media type
cannot distinguish the case. Forward 30s, back 10s, both clamped to [0, duration]
so a skip near either end cannot seek negative or read as EOF and advance.

Routed through the same spawn-then-seek_absolute path as the scrubber, because a
handoff seek re-opens the stream and must not run under the blocking lock
(DR-159). Kotlin keeps sending the opaque command; it only gains FAST_FORWARD/
REWIND in the PlaybackStateCompat so the system stops drawing skip arrows for a
control that scrubs. The remote-volume action block is deliberately untouched:
the handoff never applies to cast sessions, where skip really does mean advance.

Tests written first and watched fail (left: Advance, right: SeekTo). 706 Rust
tests pass, clippy 0, coverage 90%.

src-tauri/src/player/seek.rs

@@ -59,10 +59,149 @@ pub fn determine_video_seek_strategy(
     }
 }
 
+// The four items below are consumed by the Android MediaSessionHandler; on other
+// targets only the tests exercise them, so dead-code analysis would flag them.
+
+/// How far a lockscreen skip-forward jumps while background audio owns playback.
+#[cfg_attr(not(target_os = "android"), allow(dead_code))]
+pub const SKIP_FORWARD_SECONDS: f64 = 30.0;
+
+/// How far a lockscreen skip-back jumps while background audio owns playback.
+///
+/// Deliberately shorter than the forward jump: the back button is used to replay
+/// dialogue just missed, not to travel.
+#[cfg_attr(not(target_os = "android"), allow(dead_code))]
+pub const SKIP_BACK_SECONDS: f64 = 10.0;
+
+/// What a lockscreen skip button means for the playback that is actually running.
+#[cfg_attr(not(target_os = "android"), allow(dead_code))]
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub enum SkipAction {
+    /// Move to the next/previous queue entry — a track, or an episode.
+    Advance,
+    /// Scrub within the current item, to this absolute position in seconds.
+    SeekTo(f64),
+}
+
+/// Decide whether a lockscreen skip advances the queue or scrubs the current item.
+///
+/// Music gets queue advance, which is what the buttons look like they do. A video
+/// whose audio is playing through a background-audio handoff (UR-040) gets a
+/// relative scrub instead: there is no meaningful "next track" inside a film, and
+/// jumping to the next *episode* because the user wanted to re-hear a line is a
+/// much worse outcome than a scrub.
+///
+/// `is_background_audio` is the whole test, and it is sufficient on its own —
+/// the handoff exists only for video, and an episode played through it reports
+/// `MediaType::Audio`, so media type cannot distinguish this case (see the note
+/// at `PlayerController::auto_advance_to_next_episode`).
+///
+/// Clamped to `[0, duration]` so a skip near either end lands in the item rather
+/// than at a negative offset or past the end, which some backends treat as EOF
+/// and would turn a scrub into an unintended advance.
+///
+/// TRACES: UR-040, UR-006 | DR-201
+#[cfg_attr(not(target_os = "android"), allow(dead_code))]
+pub fn resolve_skip_action(
+    is_next: bool,
+    is_background_audio: bool,
+    position: f64,
+    duration: Option<f64>,
+) -> SkipAction {
+    if !is_background_audio {
+        return SkipAction::Advance;
+    }
+
+    let target = if is_next {
+        position + SKIP_FORWARD_SECONDS
+    } else {
+        position - SKIP_BACK_SECONDS
+    };
+
+    let clamped = match duration {
+        Some(d) if d > 0.0 => target.clamp(0.0, d),
+        _ => target.max(0.0),
+    };
+
+    SkipAction::SeekTo(clamped)
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
 
+    /// Music (no background-audio handoff) keeps queue advance on both buttons.
+    ///
+    /// TRACES: UR-006 | DR-201 | UT-194
+    #[test]
+    fn test_skip_advances_queue_for_normal_audio() {
+        assert_eq!(
+            resolve_skip_action(true, false, 42.0, Some(300.0)),
+            SkipAction::Advance
+        );
+        assert_eq!(
+            resolve_skip_action(false, false, 42.0, Some(300.0)),
+            SkipAction::Advance
+        );
+    }
+
+    /// The reported bug: in background-audio mode the lockscreen skip buttons
+    /// advanced to the next/previous episode instead of scrubbing, so trying to
+    /// re-hear a line jumped out of the film entirely.
+    ///
+    /// TRACES: UR-040 | DR-201 | UT-195
+    #[test]
+    fn test_skip_scrubs_in_background_audio_mode() {
+        assert_eq!(
+            resolve_skip_action(true, true, 100.0, Some(3600.0)),
+            SkipAction::SeekTo(130.0)
+        );
+        assert_eq!(
+            resolve_skip_action(false, true, 100.0, Some(3600.0)),
+            SkipAction::SeekTo(90.0)
+        );
+    }
+
+    /// Skipping back near the start clamps to zero rather than going negative,
+    /// which backends reject (the "Raw(-10)" class of error).
+    ///
+    /// TRACES: UR-040 | DR-201 | UT-196
+    #[test]
+    fn test_skip_back_clamps_at_start() {
+        assert_eq!(
+            resolve_skip_action(false, true, 4.0, Some(3600.0)),
+            SkipAction::SeekTo(0.0)
+        );
+    }
+
+    /// Skipping forward near the end clamps to the duration instead of running
+    /// past it, which would read as end-of-stream and advance — the very thing
+    /// this function exists to prevent.
+    ///
+    /// TRACES: UR-040 | DR-201 | UT-197
+    #[test]
+    fn test_skip_forward_clamps_at_end() {
+        assert_eq!(
+            resolve_skip_action(true, true, 3590.0, Some(3600.0)),
+            SkipAction::SeekTo(3600.0)
+        );
+    }
+
+    /// An unknown duration still scrubs, and still refuses to go negative.
+    ///
+    /// TRACES: UR-040 | DR-201 | UT-198
+    #[test]
+    fn test_skip_without_duration_still_scrubs() {
+        assert_eq!(
+            resolve_skip_action(true, true, 10.0, None),
+            SkipAction::SeekTo(40.0)
+        );
+        assert_eq!(
+            resolve_skip_action(false, true, 3.0, None),
+            SkipAction::SeekTo(0.0)
+        );
+    }
+
     /// Test video seek strategy for local files
     #[test]
     fn test_seek_strategy_local_file() {