fix(runtime): preload media on init to prevent broken first-play audio (#234)

## Summary
- Audio (and video) sounds broken/choppy on first play in the studio preview, but works fine on second play
- Root cause: `<audio>` elements default to `preload="metadata"`, which only fetches enough data to determine duration — not enough for smooth playback. When `el.play()` fires, the browser hasn't buffered the audio data yet
- The runtime now eagerly sets `preload="auto"` and calls `load()` during init, ensuring media is fully buffered before the user clicks play
- `syncRuntimeMedia` now defers `play()` on unbuffered media by registering a `canplay` listener, instead of silently swallowing the failure

## Testing
Verified with agent-browser against a 26s narration composition (soulscape-film):

```
# After fix — audio element state at init:
preload: "auto"
readyState: 4 (HAVE_ENOUGH_DATA)
buffered: 26.07s (entire file)
duration: 26.07s
```

Audio is fully buffered before any play attempt, so first-play works identically to subsequent plays.

## Files changed
- `packages/core/src/runtime/init.ts` — set `preload="auto"` + `load()` in `bindMediaMetadataListeners`
- `packages/core/src/runtime/media.ts` — defer `play()` on unbuffered media via `canplay` listener
- `packages/core/src/runtime/media.test.ts` — updated test + added unbuffered media test case

packages/core/src/runtime/init.ts

@@ -1167,6 +1167,16 @@ export function initSandboxRuntimeModular(): void {
       metadataBoundMedia.add(mediaEl);
       mediaEl.addEventListener("loadedmetadata", scheduleMetadataDurationHydration);
       mediaEl.addEventListener("durationchange", scheduleMetadataDurationHydration);
+
+      // Eagerly preload media data so audio/video is buffered before the user
+      // clicks play. Without this, the first play() call fires on un-fetched
+      // media, producing silence or choppy audio until the browser caches it.
+      if (mediaEl.preload !== "auto") {
+        mediaEl.preload = "auto";
+      }
+      if (mediaEl.readyState < HTMLMediaElement.HAVE_FUTURE_DATA) {
+        mediaEl.load();
+      }
     }
   };