fix(player): pause parent audio proxy on seek to prevent stutter loop (#890)

## Summary
- `seek()` only called `seekAll()` under parent audio ownership, leaving the `<audio>` proxy playing while the timeline froze at the new seek target.
- The periodic `mirrorTime` drift-correction (`parent-media.ts`) would then yank `currentTime` back to the timeline position every ~80ms of accumulated drift, producing an audible stutter loop while the video frame stayed frozen.
- Fix: make `seek()` symmetric with `pause()` — pause the parent proxy before seeking it.

## Repro
1. Use the player in an environment where the runtime posts `media-autoplay-blocked` (mobile / autoplay-restricted contexts), promoting audio ownership to `"parent"`.
2. Start playback with audio.
3. Click anywhere on the scrubber while playing.
4. Before: audio stutters in a short loop while the video frame is frozen.
5. After: audio cleanly pauses at the new seek position.

## Test plan
- [x] Added regression test \`seek() while playing pauses parent proxy (prevents mirrorTime stutter loop)\` in \`hyperframes-player.test.ts\`.
- [x] \`pnpm --filter @hyperframes/player test\` — 110/110 pass.
- [ ] Manual repro on a device where ownership flips to \`parent\`.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Author: terencecho

packages/player/src/hyperframes-player.test.ts

@@ -198,6 +198,26 @@ describe("HyperframesPlayer parent-frame media", () => {
     expect(mockAudio.pause).toHaveBeenCalled();
   });
 
+  it("seek() while playing pauses parent proxy (prevents mirrorTime stutter loop)", () => {
+    // Regression: previously `seek()` only called `seekAll()`, leaving the
+    // proxy playing. With the timeline frozen at the new seek target, the
+    // parent's `mirrorTime` drift-correction would yank `currentTime` back
+    // every ~80ms of accumulated drift, producing an audible audio stutter
+    // loop while the video frame stayed frozen. `seek()` must be symmetric
+    // with `pause()` for the parent-owned audio path.
+    player.setAttribute("audio-src", "https://cdn.example.com/narration.mp3");
+    document.body.appendChild(player);
+
+    player._promoteToParentProxy?.();
+    player.play();
+    expect(mockAudio.play).toHaveBeenCalled();
+    mockAudio.pause.mockClear();
+
+    player.seek(12.5);
+    expect(mockAudio.pause).toHaveBeenCalled();
+    expect(mockAudio.currentTime).toBe(12.5);
+  });
+
   it("promotion is idempotent", () => {
     player.setAttribute("audio-src", "https://cdn.example.com/narration.mp3");
     document.body.appendChild(player);

packages/player/src/hyperframes-player.ts

@@ -264,7 +264,15 @@ class HyperframesPlayer extends HTMLElement {
     this._directTimelineClock.stop();
     this._stopParentTickClock();
     this._currentTime = timeInSeconds;
-    if (this._media.audioOwner === "parent") this._media.seekAll(timeInSeconds);
+    if (this._media.audioOwner === "parent") {
+      // Pause BEFORE seek: leaving the proxy playing turns the next
+      // `mirrorTime` drift-correction tick into a perpetual seek→play→drift→seek
+      // stutter loop, where ~80ms of audio plays past the (now frozen) timeline,
+      // then mirrorTime yanks `currentTime` back to match it. Symmetric with
+      // `pause()` below.
+      this._media.pauseAll();
+      this._media.seekAll(timeInSeconds);
+    }
     this._paused = true;
     this.controlsApi?.updatePlaying(false);
     this.controlsApi?.updateTime(this._currentTime, this._duration);