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

[vanceingalls]

B5 — mute and solo, on groups and tracks

Stacks on and depends on:

• feat(studio,core): a volume and a living meter on the group row #3290 (B7 — bus strip)
• feat(engine): render grouped audio through a summed, FX-processed bus #3289 (B4 — group render)
• feat(studio,lint): carve targets voiceover groups — always, when plural #3288 (B6 — carve groups)
• feat(core): route grouped audio through a group bus in preview #3287 (B3 — group preview)
• feat(studio): group rows in the timeline, and a split disclosure #3286 (B2 — group row)
• feat(core): the audio group model — element, membership, helpers #3278 (B1 — group model)
• feat(core,studio): the character presets pitch shift unlocks #3277 (P2 — character presets)
• feat(core): pitch shift — a granular shifter as the fifth FX worklet #3276 (P1 — pitch shift)
• fix(core,studio): silence hidden audio in preview, and call it mute #3275 (A2 — audio mute)
• feat(studio): make presets the primary path into the FX rack #3274 (A1 — preset primary)

Group mute

Persisted as data-hidden on the <hf-audio-group> element itself (never written onto members — design doc §2.1's state-restoration warning). Studio action reuses B7's generic setAudioGroupAttribute (setQuiet/setLive) rather than duplicating toggleTimelineTrackHidden's shape. Render: B4 already drops every member of a data-hidden group — confirmed by a new audioMixer.test.ts case, no production change needed. Preview: a dedicated muteGain node (groupInput -> [fx] -> muteGain -> output -> master) so a mute toggle never fights scheduleVolumeLane's ramps on the same param. 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 get the strikethrough label treatment, display only.

Solo — "Hear only this"

New session-only store slice (audioSoloSlice, soloed: ReadonlySet<string> of clip/group ids, never track numbers, never serialized). Shared predicate isAudibleUnderSolo (packages/core/src/audioGroups.ts): audible only if the element or its own group is soloed. "Siblings, never ancestors" lives in the graph — solo gain is a per-element stage only, group buses are never attenuated by solo. Preview: dedicated per-element soloGain in webAudioTransport.ts, pushed via window.__hf.setAudioSolo (a direct call, not an attribute write). media.ts's HTMLMedia fallback folds the same predicate into its per-tick volume computation. Half-lit group indicator for "not soloed itself, but a member is." Exclusive-by-default toggle, ⌘/Ctrl-click to add/remove. Transport-bar banner ("Hearing only <label> — your export is not affected", Clear button).

Export-safety

The most important property: 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.

Incidental fixes

• Extracted useHydrateActiveCompPathFromUrl out of App.tsx (pre-existing, unrelated effect) to stay under the 600-line filesize cap after wiring the solo bridge in.
• 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 — that's 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)

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