feat(studio,core): reach presets and the rack from the timeline

C1: the FX button in the track/group header, and its popover — the
"reach FX from the timeline" entry point, last on purpose because it
targets a group or a single clip, never "a track" (N clips = N chains
is the ill-defined thing the design doc refuses to build).

The button (TimelineFxButton.tsx): renders on group rows and on track
rows holding exactly one audio clip, reading "FX" (or "FX n" once the
target's data-fx-chain has n enabled nodes). A multi-clip ungrouped
audio track gets a pointer instead ("Group these clips to add effects
to all of them" + a Group action) rather than silently hiding the
entry point — reuses B6's exact auto-grouping write
(useAudioGroupCarveAssignment, exposed as onGroupClips) with a minted
group id (mintGroupId, exported from useFxCarveGrouping.ts).

The popover (TimelineFxPopover.tsx, components/editor/): a thin
positioner around FxPresetMenu exactly as the property panel renders
it — same audition contract (useFxAudition), same preset-apply
computation (extracted into useApplyAudioFxPreset.ts's
applyPresetToChain, now shared with propertyPanelFxSection.tsx's own
applyPreset rather than duplicated). Escape closes without
deselecting whatever is behind it; an outside pointerdown dismisses.
Footer's "+ effect"/"Open rack ›" both select the target and hand off
to the property panel (a simplification from the step doc's two
distinct behaviors — remotely toggling the rack's own internal
"adding" state isn't plumbed anywhere, and building that plumbing
would be new UI-state wiring beyond what "reuse existing selection
dispatch" asks for).

Writes, one path per target kind, neither a new persistence mechanism:
- Group: B7/B5's existing onSetAudioGroupAttributeLive/Quiet
  (data-fx-chain, same as data-volume/data-hidden already do).
- Clip: a NEW onSetElementAttributeLive/Quiet pair
  (timelineElementFxAttribute.ts), addressed by the TimelineElement
  itself rather than the current selection. This is the one real
  architectural gap the step doc's assumption didn't survive: the
  property panel's onSetAttributeQuiet closes over domEditSelection,
  so writing a clip that isn't already selected has no synchronous
  path through it. Extracted the shared live-patch-then-persist core
  (persistElementAttribute, timelineEditingHelpers.ts) out of both
  this new path and the existing setAudioGroupAttribute, which the
  fallow duplication gate flagged as a 66-line clone on first pass —
  now a single ~50-line core parameterized by patchLive/readLive, with
  each caller a ~15-line wrapper resolving its own patch target
  (buildPatchTarget({domId}) for a group, buildPatchTarget(element)
  for an arbitrary clip) and live-DOM lookup.

Data plumbing: HfAudioGroup.fxChain (already on the B1 model) mirrored
onto TimelineElement.audioGroupFxChain (timelineDOM.ts's groupInfoFor
cache) and TimelineTrackGroupInfo.fxChain (useTimelineTrackDerivations.ts),
alongside the existing volume/hidden mirrors.

Deferred: the property panel's own rack doesn't (yet) expose a way to
remotely force its add-menu open, so "+ effect" and "Open rack ›"
converge on the same navigation rather than the step doc's two
distinct ones. A grouped multi-clip track (some clips already carry
data-audio-group) gets neither the chain button nor the pointer —
its members' own per-clip FX buttons still work individually, and the
group's own FX button on TimelineGroupHeader covers the group level.

Gates: bun run build clean; packages/studio full suite 4286/4304 (18
pre-existing todo, up from 4276/4294 — 10 new tests, 0 regressions);
new TimelineFxPopover.test.tsx (6) + TimelineFxButton.test.tsx (4)
cover exactly-one-write-per-apply, hover-audition-reverts-on-leave,
Escape-without-deselecting, outside/inside pointerdown dismissal, and
the group-pointer's Group action; oxfmt/oxlint clean on all 22 touched
files; fallow clean (0 new dead-code/unused-export/duplication
findings — the pointer test caught during the first commit attempt).

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

packages/studio/src/player/lib/timelineDOM.ts

@@ -70,22 +70,28 @@ function resolveClipTag(clip: ClipManifestClip): string {
 
 // One `<hf-audio-group>` scan per document, not per clip — resolveAudioGroups
 // walks the whole tree, and a parse touches every clip in it.
-const groupInfoCache = new WeakMap<
-  Document,
-  Map<string, { label: string; volume: number; hidden: boolean }>
->();
+interface GroupInfo {
+  label: string;
+  volume: number;
+  hidden: boolean;
+  fxChain?: string;
+}
 
-function groupInfoFor(
-  doc: Document | null | undefined,
-  groupId: string,
-): { label: string; volume: number; hidden: boolean } {
+const groupInfoCache = new WeakMap<Document, Map<string, GroupInfo>>();
+
+function groupInfoFor(doc: Document | null | undefined, groupId: string): GroupInfo {
   if (!doc) return { label: groupId, volume: 1, hidden: false };
   let info = groupInfoCache.get(doc);
   if (!info) {
     info = new Map(
       resolveAudioGroups(doc).map((group) => [
         group.id,
-        { label: group.label, volume: group.volume, hidden: group.hidden },
+        {
+          label: group.label,
+          volume: group.volume,
+          hidden: group.hidden,
+          ...(group.fxChain ? { fxChain: group.fxChain } : {}),
+        },
       ]),
     );
     groupInfoCache.set(doc, info);
@@ -171,6 +177,7 @@ export function createTimelineElementFromManifestClip(params: {
       entry.audioGroupLabel = info.label;
       entry.audioGroupVolume = info.volume;
       entry.audioGroupHidden = info.hidden;
+      if (info.fxChain) entry.audioGroupFxChain = info.fxChain;
     }
     const fxChain = hostEl.getAttribute("data-fx-chain");
     if (fxChain) entry.fxChain = fxChain;
@@ -397,6 +404,7 @@ export function parseTimelineFromDOM(doc: Document, rootDuration: number): Timel
       entry.audioGroupLabel = domGroupInfo.label;
       entry.audioGroupVolume = domGroupInfo.volume;
       entry.audioGroupHidden = domGroupInfo.hidden;
+      if (domGroupInfo.fxChain) entry.audioGroupFxChain = domGroupInfo.fxChain;
     }
 
     // Sub-compositions