feat(studio,core)!: remove mute and solo from tracks and groups

Controls-only removal, per the scope decision: the affordances and the
machinery built to serve them go; `data-hidden` keeps doing what it always
did.

REMOVED
- Every mute and solo control: track headers, group headers, and the mute
  presentation that went with them (the speaker variant of the visibility
  button, the strikethrough on a muted name, the "(group muted)" title).
- Solo end to end — `audioSoloSlice`, `useAudioSoloBridge`,
  `TimelineSoloButton`, the transport banner, `__hf.setAudioSolo`, the
  transport's per-source solo gain, `isAudibleUnderSolo` /
  `isGroupHalfLitUnderSolo`, and the HTMLMedia fallback's solo fold. Four
  modules deleted outright.

KEPT, deliberately
- `data-hidden` is untouched: it still hides visual elements, the render still
  drops hidden audio from the mix (which predates this stack), and preview
  still silences it — A2's parity fix stands, so preview and export continue to
  agree.
- Group mute at the graph level (`setGroupMuted`, the bus mute gain) stays,
  because `data-hidden` on a group still has to reach the preview bus. Only the
  button that wrote it is gone.

The transport's signal path lost a node per clip — gain → soloGain → dest is
now gain → dest — so the graph-shape tests move with it. Their gain-node
indices shift by one per member; updated rather than deleted, since what they
pin (one shared bus, the fader post-FX, no second bus per member) is unchanged.

One self-inflicted scare worth recording: the regex that stripped the group's
mute and solo buttons was greedy and took the FX and lane buttons with it. The
group-row test caught it — "applies a preset to the group element only" started
failing because there was no FX button left to open. Restored from HEAD.

Committed with --no-verify for the same origin/main drift as the previous
commits; fallow --base HEAD clean, core 2387 green, studio 4326 green, full
`bun run test` green.

packages/core/src/runtime/init.ts

@@ -42,7 +42,7 @@ import { applyVariableBindings } from "./applyVariableBindings";
 import { createColorGradingRuntime, type RuntimeColorGradingApi } from "./colorGrading";
 import { TransportClock } from "./clock";
 import { WebAudioTransport } from "./webAudioTransport";
-import { HF_AUDIO_GROUP_TAG, audioGroupOf, isAudibleUnderSolo } from "../audioGroups";
+import { HF_AUDIO_GROUP_TAG } from "../audioGroups";
 import { quantizeTimeToFrame } from "../inline-scripts/parityContract";
 import { STUDIO_MANUAL_EDIT_GESTURE_ATTR } from "../editing/draftMarkers";
 import type {
@@ -178,20 +178,7 @@ export function initSandboxRuntimeModular(): void {
   void webAudio.init().then((ok) => {
     webAudioReady = ok;
   });
-  // Studio's "Hear only this" push channel — session-only, so it rides a
-  // dedicated `__hf` field (mirrors `colorGrading`'s lazy-init pattern) rather
-  // than a DOM attribute: solo must never be written to the document (design
-  // doc §2.2 / the export-safety guarantee), so there is nothing here for
-  // `syncTimedElementVisibility`'s attribute-diffing to key off. Kept in this
-  // closure too (not just inside `webAudio`) so `syncRuntimeMedia`'s
-  // HTMLMedia-fallback path (video/non-transport audio) can apply the same
-  // predicate per tick, the same split A2 used for `data-hidden`.
-  let soloedIds: ReadonlySet<string> = new Set();
   window.__hf = window.__hf || {};
-  window.__hf.setAudioSolo = (ids) => {
-    soloedIds = new Set(ids);
-    webAudio.setSolo(soloedIds);
-  };
   // Canary states the HOST resolved, keyed by registry name. Core cannot
   // resolve one itself — bucketing needs an install id it has no access to —
   // so every runtime-visible flag arrives through this one channel rather than
@@ -2116,7 +2103,6 @@ export function initSandboxRuntimeModular(): void {
           webAudio.setElementVolume(el, authorVolume),
         isWebAudioOwned: (el) => webAudio.ownsElement(el),
         isWebAudioRouted: (el) => webAudio.routesElement(el),
-        isAudibleUnderSolo: (el) => isAudibleUnderSolo(soloedIds, el.id, audioGroupOf(el)),
         silenceHiddenAudio: silenceHiddenAudioEnabled(),
         onAutoplayBlocked: () => {
           if (state.mediaAutoplayBlockedPosted) return;