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.test.ts

@@ -0,0 +1,61 @@
+import { describe, it, expect } from "vitest";
+import {
+  computeHandoffPosition,
+  initialHandoffState,
+  shouldEnterBackgroundAudio,
+  shouldExitBackgroundAudio,
+  type BackgroundAudioState,
+} from "./backgroundAudioHandoff";
+
+// TRACES: UR-040 | DR-052 | UT-060
+
+describe("backgroundAudioHandoff", () => {
+  describe("computeHandoffPosition", () => {
+    it("sums element time and transcode seekOffset (absolute position)", () => {
+      // Transcoded HLS resets element time to 0 after a reload; seekOffset carries
+      // the cumulative offset. The audio stream must resume at the absolute pos.
+      expect(computeHandoffPosition(12, 180)).toBe(192);
+    });
+
+    it("handles a direct stream with no offset", () => {
+      expect(computeHandoffPosition(45, 0)).toBe(45);
+    });
+
+    it("never returns a negative position", () => {
+      expect(computeHandoffPosition(-5, 0)).toBe(0);
+    });
+  });
+
+  describe("shouldEnterBackgroundAudio", () => {
+    it("enters when toggle is on and not already handed off", () => {
+      expect(shouldEnterBackgroundAudio(true, initialHandoffState)).toBe(true);
+    });
+
+    it("does not enter when the toggle is off", () => {
+      expect(shouldEnterBackgroundAudio(false, initialHandoffState)).toBe(false);
+    });
+
+    it("does not double-enter when already active", () => {
+      const active: BackgroundAudioState = { active: true, wasPlaying: true };
+      expect(shouldEnterBackgroundAudio(true, active)).toBe(false);
+    });
+  });
+
+  describe("shouldExitBackgroundAudio", () => {
+    it("exits when a handoff is active", () => {
+      const active: BackgroundAudioState = { active: true, wasPlaying: false };
+      expect(shouldExitBackgroundAudio(active)).toBe(true);
+    });
+
+    it("does not exit when no handoff happened", () => {
+      expect(shouldExitBackgroundAudio(initialHandoffState)).toBe(false);
+    });
+
+    it("exits even if the toggle was turned off while backgrounded", () => {
+      // shouldExit ignores the toggle by design, so turning it off mid-background
+      // still returns cleanly to video on foreground.
+      const active: BackgroundAudioState = { active: true, wasPlaying: true };
+      expect(shouldExitBackgroundAudio(active)).toBe(true);
+    });
+  });
+});