feat(studio,core): mute groups, and hear-only-this that cannot reach the export

B5: mute and solo, on groups and tracks (track mute already shipped by A2 —
nothing to build there).

Group mute — persisted as data-hidden on the <hf-audio-group> element itself
(never written onto members, per design doc §2.1's state-restoration
warning). Studio action reuses B7's generic setAudioGroupAttribute
(setQuiet/setLive split) rather than duplicating toggleTimelineTrackHidden's
shape — same one-atomic-patch/one-undo-entry contract, already built for
exactly this purpose. Render: B4 already drops every member of a
data-hidden group (confirmed by a new audioMixer.test.ts case — no
production change needed there). Preview: a dedicated muteGain node
(groupInput -> [fx] -> muteGain -> output -> master) so a mute toggle
never fights scheduleVolumeLane's ramps on the same param — the same
hazard B7's volume fader was split out to avoid. Mid-playback toggles
sync via a new syncAudioGroupMute pass in init.ts (a group carries no
data-start, so it's invisible to the existing visibility-node query).
Members of a muted group render the strikethrough label treatment
(TimelineTrackPlainHeader's isGroupMuted, sourced from
TimelineElement.audioGroupHidden) — display only, no attribute touched.

Solo — "Hear only this": a new session-only store slice (audioSoloSlice,
soloed: ReadonlySet<string> of clip/group ids, never track numbers, never
serialized). Predicate (isAudibleUnderSolo, packages/core/src/audioGroups.ts
so both the store and the preview transport share one definition): an
element is audible while any solo is active only if it or its own group is
soloed. "Siblings, never ancestors" lives in the graph, not the predicate —
solo gain is a per-element stage only; group buses are never attenuated by
solo, so a soloed member's path through its group stays open by
construction. Preview: a dedicated per-element soloGain in
webAudioTransport.ts (parallel to the mute mechanics), pushed via
window.__hf.setAudioSolo — a direct call, not an attribute write, so it
can't ride the visibility-diff path mute uses. media.ts's HTMLMedia
fallback folds the same predicate into its per-tick volume computation
(the same seam A2 used for data-hidden). Half-lit group indicator
(isGroupHalfLitUnderSolo) for "not soloed itself, but a member is".
Exclusive-by-default toggle, ⌘/Ctrl-click to add/remove, TimelineSoloButton
(⌗) beside mute on both track and group headers. Transport-bar banner
("Hearing only <label> — your export is not affected", Clear button) added
in PlayerControls.tsx, reading labels straight off the live preview DOM.

Export-safety, the most important property here: toggling/adding/clearing
solo never calls setAttribute/removeAttribute on any element and never
invokes the project save path (both asserted directly via spies in
audioSoloSlice.test.ts) — solo cannot reach an export by construction, not
by convention.

Also: extracted useHydrateActiveCompPathFromUrl out of App.tsx (a
pre-existing, unrelated effect) to stay under the 600-line filesize cap
after wiring useAudioSoloBridge in; and fixed a circular dependency the
solo-banner wiring introduced (useAudioSoloBridge.ts now imports
usePlayerStore from its concrete module instead of the player/ barrel,
which re-exports PlayerControls.tsx — the barrel path is what closed the
cycle).

Gates: bun run build clean; packages/core full suite 2379/2379; packages/
studio full suite 4276/4294 (18 pre-existing todo); packages/engine
audioMixer.grouping.test.ts 5/5; oxfmt/oxlint clean on all 23 touched
files; fallow clean (0 new circular deps, 0 new filesize/complexity
findings).

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

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

@@ -1,15 +1,12 @@
-import { Eye, EyeSlash, SpeakerHigh, SpeakerSlash } from "@phosphor-icons/react";
 import type { GsapAnimation } from "@hyperframes/core/gsap-parser";
-import { isCanaryEnabled } from "../../telemetry/canary";
-import { Music } from "../../icons/SystemIcons";
-import type { TimelineElement } from "../store/playerStore";
+import { usePlayerStore, type TimelineElement } from "../store/playerStore";
+import { VisibilityButton, PlainTrackHeader } from "./TimelineTrackPlainHeader";
 import type { TimelineEditCallbacks } from "./timelineCallbacks";
 import { getTimelinePropertyLanes } from "./TimelinePropertyLanes";
 import { groupAutomationLanes } from "./automationLaneData";
 import { AUTOMATION_LANE_H } from "./automationLaneHeight";
 import { clipTimingStart } from "../../hooks/gsapShared";
 import { LayerDisclosureRow } from "./LayerDisclosureRow";
-import { TrackClipCount } from "./TrackClipCount";
 import { LABEL_COL_W, LANE_H, getTimelineLaneTop } from "./timelineLayout";
 import type { TimelineTheme } from "./timelineTheme";
 import {
@@ -62,109 +59,6 @@ interface TimelineTrackHeaderProps {
   onSeek?: (time: number) => void;
 }
 
-// Audio tracks say "Mute", not "Hide" — the eye IS mute for sound-only rows.
-// Gated: the relabel ships behind the canary, unlike the preview fix.
-function visibilityButtonLabel(showAsMute: boolean, hidden: boolean, suffix: string): string {
-  if (showAsMute) return hidden ? "Muted" : "Mute";
-  return hidden ? `Show track${suffix}` : `Hide track${suffix}`;
-}
-
-function visibilityButtonIcon(showAsMute: boolean, hidden: boolean) {
-  const Icon = showAsMute ? (hidden ? SpeakerSlash : SpeakerHigh) : hidden ? EyeSlash : Eye;
-  return <Icon size={14} weight="bold" aria-hidden="true" />;
-}
-
-function VisibilityButton({
-  hidden,
-  trackNumber,
-  trackDisplayNumber,
-  visible,
-  isAudioTrack,
-  onToggle,
-}: {
-  hidden: boolean;
-  trackNumber: number;
-  trackDisplayNumber: number | null;
-  visible: boolean;
-  isAudioTrack?: boolean;
-  onToggle: TimelineEditCallbacks["onToggleTrackHidden"];
-}) {
-  if (!visible) return <span aria-hidden="true" className="h-6 w-6 shrink-0" />;
-  // Display number in the text, real key in the callback. The two must not be
-  // conflated in either direction.
-  const suffix = trackDisplaySuffix(trackDisplayNumber);
-  const showAsMute = Boolean(isAudioTrack) && isCanaryEnabled("audio-track-mute");
-  const label = visibilityButtonLabel(showAsMute, hidden, suffix);
-  return (
-    <button
-      type="button"
-      aria-label={label}
-      title={label}
-      className={`flex h-6 w-6 shrink-0 items-center justify-center rounded border-0 bg-transparent p-0 transition-colors focus-visible:outline focus-visible:outline-1 focus-visible:outline-offset-[-1px] focus-visible:outline-[#3CE6AC] ${
-        hidden ? "text-[#3CE6AC] hover:text-white" : "text-white/35 hover:text-white/75"
-      }`}
-      onPointerDown={(event) => event.stopPropagation()}
-      onClick={(event) => {
-        event.stopPropagation();
-        void onToggle?.(trackNumber, !hidden);
-      }}
-    >
-      {visibilityButtonIcon(showAsMute, hidden)}
-    </button>
-  );
-}
-
-// The header a track gets when it has no keyframe clip to disclose: label, clip
-// count, eye. Not deprecated — it is the live path for every track without lanes.
-function PlainTrackHeader({
-  trackNumber,
-  trackDisplayNumber,
-  trackLabel,
-  clipCount,
-  showTrackLabel,
-  isTrackHidden,
-  isAudioTrack,
-  onToggleTrackHidden,
-}: Pick<
-  TimelineTrackHeaderProps,
-  | "trackNumber"
-  | "trackDisplayNumber"
-  | "trackLabel"
-  | "clipCount"
-  | "isTrackHidden"
-  | "isAudioTrack"
-  | "onToggleTrackHidden"
-> & { showTrackLabel: boolean }) {
-  return (
-    <>
-      {isAudioTrack && (
-        <Music size={12} weight="fill" aria-hidden="true" className="text-white/35" />
-      )}
-      {showTrackLabel && (
-        <span
-          className={`min-w-0 flex-1 truncate text-[11px] ${
-            isAudioTrack && isTrackHidden && isCanaryEnabled("audio-track-mute")
-              ? "line-through"
-              : ""
-          }`}
-          title={trackLabel}
-        >
-          {trackLabel}
-        </span>
-      )}
-      {showTrackLabel && <TrackClipCount clipCount={clipCount} />}
-      <VisibilityButton
-        hidden={isTrackHidden}
-        trackNumber={trackNumber}
-        trackDisplayNumber={trackDisplayNumber}
-        visible
-        isAudioTrack={isAudioTrack}
-        onToggle={onToggleTrackHidden}
-      />
-    </>
-  );
-}
-
 // Figma layout: prev-keyframe ‹, the add/remove toggle (children), next ›.
 function PropertyGroupNavigation({
   navigation,
@@ -478,6 +372,13 @@ export function TimelineTrackHeader({
   // left an audio clip's envelopes unreachable, since the track could not expand.
   const disclosable = lanes.length > 0 || automationRows.length > 0;
   const isKeyframeLayer = !!keyframeClip && disclosable;
+  // Solo is per-clip/per-group, never per track (design doc §2.2) — this header
+  // acts on the track's first clip as a pragmatic stand-in for "this track",
+  // the same simplification the mute button doesn't need to make (it patches
+  // every clip on the track at once).
+  const soloTargetId = trackElements[0] ? (trackElements[0].key ?? trackElements[0].id) : null;
+  const soloed = usePlayerStore((s) => s.soloed);
+  const toggleSolo = usePlayerStore((s) => s.toggleSolo);
 
   return (
     <div
@@ -505,6 +406,9 @@ export function TimelineTrackHeader({
           showTrackLabel={showTrackLabel}
           isTrackHidden={isTrackHidden}
           isAudioTrack={isAudioTrack}
+          isGroupMuted={trackElements.some((el) => el.audioGroupHidden)}
+          isSoloed={soloTargetId !== null && soloed.has(soloTargetId)}
+          onToggleSolo={soloTargetId ? (options) => toggleSolo(soloTargetId, options) : undefined}
           onToggleTrackHidden={onToggleTrackHidden}
         />
       ) : (