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/hooks/useHydrateActiveCompPathFromUrl.ts

@@ -0,0 +1,36 @@
+import { useEffect } from "react";
+import type { MutableRefObject } from "react";
+import { normalizeStudioCompositionPath, type StudioUrlState } from "../utils/studioUrlState";
+
+/**
+ * One-time hydration of `activeCompPath` from the initial URL state, once the
+ * file tree has loaded (a path that isn't in the tree yet can't be
+ * validated). Runs exactly once — `hydrated` flips true whether or not the
+ * URL named a valid path, so a later file-tree change never re-fires it.
+ */
+export function useHydrateActiveCompPathFromUrl({
+  hydrated,
+  fileTreeLoaded,
+  fileTree,
+  initialUrlStateRef,
+  setActiveCompPath,
+  setHydrated,
+}: {
+  hydrated: boolean;
+  fileTreeLoaded: boolean;
+  fileTree: string[];
+  initialUrlStateRef: MutableRefObject<StudioUrlState>;
+  setActiveCompPath: (updater: (current: string | null) => string | null) => void;
+  setHydrated: (value: boolean) => void;
+}): void {
+  useEffect(() => {
+    if (hydrated) return;
+    if (!fileTreeLoaded) return;
+    const nextCompPath = normalizeStudioCompositionPath(
+      initialUrlStateRef.current.activeCompPath,
+      fileTree,
+    );
+    setActiveCompPath((current) => (current === nextCompPath ? current : nextCompPath));
+    setHydrated(true);
+  }, [hydrated, fileTree, fileTreeLoaded, initialUrlStateRef, setActiveCompPath, setHydrated]);
+}