Background-audio handoff for video + repository/player refactor

Hand video playback off to a native audio-only stream when the app is
backgrounded or locked, with no on-device video decode (UR-040). Adds
player_enter/exit_background_audio commands, an audio-only stream URL
for video items across the repository layer, and the frontend handoff
state machine wired into VideoPlayer. Includes accompanying
repository/offline/player refactoring and regenerates the traceability
matrix.

src/lib/components/player/backgroundAudioHandoff.ts

@@ -0,0 +1,57 @@
+/**
+ * Pure helpers for the video → background-audio handoff (UR-040).
+ *
+ * TRACES: UR-040 | DR-052 | UT-060
+ *
+ * Kept free of Svelte/DOM so the handoff arithmetic and state transitions are
+ * unit-testable without mounting the player. The component
+ * (VideoPlayer.svelte) owns the actual `<video>` teardown and IPC calls.
+ */
+
+/**
+ * Absolute playback position to resume the audio stream at.
+ *
+ * Transcoded HLS playback tracks time as `videoElement.currentTime + seekOffset`
+ * (the element resets to 0 after each transcode reload; `seekOffset` carries the
+ * cumulative offset). Background audio must resume at that ABSOLUTE position, so
+ * both terms are summed here — mirroring the `effectiveTime` used elsewhere in
+ * the player.
+ */
+export function computeHandoffPosition(elementCurrentTime: number, seekOffset: number): number {
+  const pos = elementCurrentTime + seekOffset;
+  return pos > 0 ? pos : 0;
+}
+
+/**
+ * The handoff state. `wasPlaying` is captured on the way out so play/pause is
+ * restored when the app returns to the foreground.
+ */
+export interface BackgroundAudioState {
+  active: boolean;
+  wasPlaying: boolean;
+}
+
+export const initialHandoffState: BackgroundAudioState = {
+  active: false,
+  wasPlaying: false,
+};
+
+/**
+ * Whether a background signal should trigger the audio handoff right now.
+ * Only when the toggle is on and we're not already handed off.
+ */
+export function shouldEnterBackgroundAudio(
+  toggleOn: boolean,
+  state: BackgroundAudioState
+): boolean {
+  return toggleOn && !state.active;
+}
+
+/**
+ * Whether a foreground signal should trigger the return to WebView video.
+ * Only when we actually handed off (regardless of the current toggle value, so
+ * turning the toggle off while backgrounded still returns cleanly).
+ */
+export function shouldExitBackgroundAudio(state: BackgroundAudioState): boolean {
+  return state.active;
+}