fix(player): correct playback rate for direct-timeline and audio-clock paths (#849)

## Summary

- **Direct-timeline path** (GSAP compositions with `window.__timelines`): The player drives these via `DirectTimelineAdapter`, bypassing postMessage entirely. Rate changes sent `set-playback-rate` to the iframe but had no receiver — GSAP's `timeScale()` was never called. Fix: add optional `timeScale?` to `DirectTimelineAdapter` and call `this._directTimelineAdapter?.timeScale?.(rate)` in `attributeChangedCallback`. GSAP timelines expose `timeScale` natively, no composition changes required.

- **Audio-clock path** (compositions with audio): Three bugs caused `TransportClock` to always run at 1x when an audio element or WebAudio context drove the clock:
  1. `schedulePlayback` was called without the `playbackRate` arg (defaulted to 1).
  2. `onSetPlaybackRate` and `player.setPlaybackRate` didn't call `webAudio.setRate()`.
  3. `TransportClock.attachAudioSource` divided by `this._rate` instead of `el.playbackRate`, cancelling the rate multiplier.

- Adds 2 regression tests to `clock.test.ts` covering the corrected audio-clock formula.

## Test plan

- [ ] Unit tests: `bun run --cwd packages/core test` — 861/861 pass
- [ ] Browser verification (Playwright headless, GSAP direct-timeline composition):
  - 1x speed → ratio 0.972 ✓
  - 2x speed → ratio 1.965 ✓
  - 0.5x speed → ratio 0.490 ✓

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

packages/player/src/hyperframes-player.ts

@@ -177,6 +177,7 @@ class HyperframesPlayer extends HTMLElement {
         const rate = parseFloat(val || "1");
         this._media.updatePlaybackRate(rate);
         this._sendControl("set-playback-rate", { playbackRate: rate });
+        this._directTimelineAdapter?.timeScale?.(rate);
         this.controlsApi?.updateSpeed(rate);
         this.dispatchEvent(new Event("ratechange"));
         break;

packages/player/src/timeline-adapters.ts

@@ -24,6 +24,8 @@ export interface DirectTimelineAdapter {
   seek: (timeInSeconds: number) => unknown;
   play: () => unknown;
   pause: () => unknown;
+  /** Optional: set playback rate (e.g. GSAP's timeScale). Called when the player's playbackRate changes. */
+  timeScale?: (scale: number) => unknown;
 }
 
 export type PlaybackDurationAdapter =