fix(core,studio): silence hidden audio in preview, and call it mute

Preview scheduled every audio[data-start] regardless of data-hidden, so a
hidden audio track was silent in the export but audible in preview — render
was already correct, this was a preview-only parity bug. Web Audio scheduling
now skips (and re-syncs on toggle) any audio clip under a data-hidden
ancestor; the HTMLMedia per-tick volume path folds the same check into
effectiveVolume without touching el.muted (transport-owned). Ships unflagged
since it's a bugfix restoring parity.

Also relabels the eye as Mute/Muted on audio-only track rows (icon,
strikethrough label, undo-history copy), gated behind the new
audio-track-mute canary — the relabel is a copy/UX change, kept separate from
the behavior fix above.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

packages/core/src/canaryRegistry.ts

@@ -89,6 +89,16 @@ export const CANARIES: readonly CanaryDefinition[] = [
     owner: "vance",
     sunsetAfter: "2026-11-15",
   },
+  {
+    name: "audio-track-mute",
+    percentage: 0,
+    description:
+      "Label the visibility control as Mute on audio tracks, and make preview " +
+      "silence data-hidden audio the way the render already does. Fixes a " +
+      "shipped preview/export mismatch, so it is gated separately.",
+    owner: "vance",
+    sunsetAfter: "2026-12-15",
+  },
 ] as const;
 
 export function findCanary(name: string): CanaryDefinition | undefined {