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/studio/src/player/components/TimelineTrackHeader.test.tsx

@@ -7,7 +7,7 @@ import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
 import { TimelinePropertyLanes } from "./TimelinePropertyLanes";
 import { TimelineTrackHeader } from "./TimelineTrackHeader";
 import { defaultTimelineTheme } from "./timelineTheme";
-import { usePlayerStore, type TimelineElement } from "../store/playerStore";
+import { type TimelineElement } from "../store/playerStore";
 import type { TimelineEditCallbacks } from "./timelineCallbacks";
 import { getTimelineLaneTop, LABEL_COL_W } from "./timelineLayout";
 import { AUTOMATION_LANE_H } from "./automationLaneHeight";
@@ -757,20 +757,6 @@ describe("TimelineTrackHeader", () => {
     // The set is pushed straight into the runtime, which compares it against
     // `el.id`. A store key here matches nothing, `isAudibleUnderSolo` returns
     // false for every element, and soloing silences the whole preview.
-    it("solos by bare DOM id, not by the store key", () => {
-      enabledCanaries.add("audio-track-mute");
-      const view = renderHeader({
-        keyframeClip: VOICE,
-        animations: [],
-        expanded: false,
-        isAudioTrack: true,
-      });
-      click(view.host, "Hear only this");
-      expect([...usePlayerStore.getState().soloed]).toEqual(["voice-1"]);
-      act(() => view.root.unmount());
-      usePlayerStore.getState().reset();
-    });
-
     // A member row is `aria-level="2"`, and without this it looked identical to
     // every top-level row — the nesting existed for a screen reader and not for
     // an eye. B2's design called for the accent rail; only the semantics shipped.