fix(runtime,player): replay bridge state on iframe ready to repair race

The audio-locked attribute was correctly setting `muted = true` and posting
`set-muted` to the iframe runtime, but on warm-cache reloads of claude.ai
and inside the Claude desktop Electron client, the iframe finishes loading
*after* the parent has already sent control messages — the iframe runtime's
postMessage listener isn't installed yet, so the messages are silently
dropped. Audio plays unmuted with no UI to recover.

Confirmed via:
- "First open" on claude.ai: cold cache, iframe slow → listener up before
  `set-muted` lands → audio muted ✅
- "Hard refresh" on claude.ai: warm cache, iframe fast → listener up after
  message arrives → message lost → audio plays ❌
- Claude desktop: Electron renderer consistently fast → race always loses
  → audio plays ❌

Fix: add a `{source: "hf-preview", type: "ready"}` event the runtime emits
once `installRuntimeControlBridge` has registered the listener. The player
listens for it and replays current bridge state (`set-muted`, `set-volume`,
`set-playback-rate`). Pre-ready messages are now safe to send — they'll be
replayed once the runtime can receive them.

The replay is idempotent — re-asserting defaults is a no-op — so it's also
safe across iframe reloads (new runtime instance emits ready again).

Tests: 6 new (1 bridge: ready posted on install; 5 player: replays muted /
volume / playback-rate / audio-locked-forced-mute / handles second ready /
ignores ready from wrong source). Suites green: core 1387, player 137.

Refs:
- Investigation: heygen-com/hyperframes#1300 (UA-fallback attempt — unrelated
  to actual root cause)
- claude.ai-web.log analysis revealed cross-origin iframe + race condition,
  not attribute stripping as originally hypothesized

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

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/core/src/runtime/README.md

@@ -32,6 +32,11 @@ postMessage:
 - runtime -> parent events:
   - `source: "hf-preview"`
   - `type: "state"` and `type: "timeline"`
+  - `type: "ready"` — emitted once when `installRuntimeControlBridge` registers
+    the control-message listener. The parent uses it to replay current playback
+    state (`set-muted`, `set-volume`, `set-playback-rate`) so any control
+    message sent before the listener was installed isn't lost. Emitted again on
+    every iframe reload because the new runtime instance starts with no state.
 
 Determinism baseline: