feat(player): add volume/mute controls (#651)

* feat(player): add volume/mute controls to the player

Adds a mute toggle button and volume slider to the controls bar,
positioned between the time display and speed selector. The slider
expands on hover for a compact default footprint.

- `volume` attribute/property (0–1, clamped) with `volumechange` event
- `muted` attribute now syncs to the controls UI (icon updates)
- Three volume icons: high, low, muted — updates reactively
- Volume forwarded to parent-frame audio proxies and iframe runtime
  via `set-volume` postMessage control
- 9 new tests covering volume clamping, events, controls rendering,
  mute toggle, and iframe message forwarding

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* fix(player): wire set-volume through runtime bridge + address review feedback

Addresses the blocker from PR review: the iframe runtime now handles
`set-volume` messages end-to-end (types → bridge → init → media sync).

Runtime side:
- Add `set-volume` to RuntimeBridgeControlAction union
- Add `volume` field to RuntimeBridgeControlMessage
- Handle `set-volume` in bridge.ts with [0,1] clamping
- Store bridgeVolume in RuntimeState, apply to media elements
- syncRuntimeMedia composes userVolume × clip author volume

Player side:
- Muted toggle now dispatches `volumechange` (HTML5 spec compliance)
- Volume slider auto-unmutes when scrubbed above 0 while muted
- Touch support on volume slider (touchstart/move/end)

Tests: 5 new (3 bridge, 2 media)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* fix(player): add ARIA keyboard controls, fix icon collision and clipVolume parity

- Volume slider: role="slider", aria-label, aria-valuemin/max/now,
  tabindex=0, arrow key support (5% steps, auto-unmutes)
- Volume=0 unmuted now shows low-volume icon instead of muted icon
- Fix clipVolume divergence: init.ts uses Number.isFinite() matching
  media.ts semantics (preserves data-volume="0")
- 3 new tests: ARIA attributes, volumechange on mute, icon collision

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/core/src/runtime/bridge.ts

@@ -5,6 +5,7 @@ type BridgeDeps = {
   onPause: () => void;
   onSeek: (frame: number, seekMode: "drag" | "commit") => void;
   onSetMuted: (muted: boolean) => void;
+  onSetVolume: (volume: number) => void;
   onSetMediaOutputMuted: (muted: boolean) => void;
   onSetPlaybackRate: (rate: number) => void;
   onEnablePickMode: () => void;
@@ -40,6 +41,10 @@ export function installRuntimeControlBridge(deps: BridgeDeps): (event: MessageEv
       deps.onSetMuted(Boolean(data.muted));
       return;
     }
+    if (action === "set-volume") {
+      deps.onSetVolume(Math.max(0, Math.min(1, Number(data.volume ?? 1))));
+      return;
+    }
     if (action === "set-media-output-muted") {
       deps.onSetMediaOutputMuted(Boolean(data.muted));
       return;