feat(studio,core): a volume and a living meter on the group row

B7: the group bus strip — droppable, and deliberately minimal per the
casual-user design constraints (groups doc §5): a volume slider, a level
bar that moves with the sound, and the words "Too loud" when it clips. No
dB numbers, no peak-hold readout, no routing row.

Transport (core): groupInput() now routes each group through input -> [FX
chain or dry passthrough] -> output -> master, with one AnalyserNode per
group tapped off `output` (post-FX, so the meter reads what the bus
actually outputs) — fftSize 256, level not spectrum. groupLevel(groupId)
returns RMS-ish level 0..1 + a clipped flag off a reused per-group buffer
(no per-frame allocation), or null when the group is idle/unknown. The
runtime posts group-levels messages only while playing, piggybacking the
existing message channel rather than adding a new poll loop.

Studio: groupLevels.ts is a plain pub-sub store (mirrors liveTime.ts's
shape) fed by useTimelinePlayer's message handler via
parseGroupLevelsMessage; useGroupLevel throttles re-renders to ~33ms.
TimelineGroupBusStrip renders in the group row's own `∿` lane area
(STRIP_H, already sized in B2's row-height pipeline) — drag writes live
via onSetAudioGroupAttributeLive, release commits one undo entry via
onSetAudioGroupAttributeQuiet (packages/studio/src/hooks/
timelineAudioGroupVolume.ts, extracted from timelineTrackVisibility.ts to
stay under the 600-line cap; mirrors FxParamRow's live/commit split).
"Too loud" holds for ~2s after the last clipped block, tracked in the
component, not the transport. volumeByGroup mirrors labelByGroup in
useTimelineTrackDerivations.ts so the strip's slider round-trips the
group's own data-volume.

Fixed two pre-existing group-routing tests in webAudioTransport.test.ts
that hardcoded gain-node creation order/count — B7 inserts an extra
`output` gain node between the group's input and master (for the meter to
tap), which shifted node indices the tests asserted on directly.

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

packages/core/src/runtime/init.ts

@@ -2140,6 +2140,25 @@ export function initSandboxRuntimeModular(): void {
     scheduleRootStageLayoutDiagnostics();
   };
 
+  /** One meter reading per group with an active member — polled from the
+   *  transport's analyser, not the DOM, so an idle group (never played, no
+   *  matching `<hf-audio-group>`) is simply absent rather than reported as
+   *  zero. Cheap when nothing is grouped: `groupIds()` is empty. */
+  const postGroupLevels = () => {
+    const groupIds = webAudio.groupIds();
+    if (groupIds.length === 0) return;
+    const levels = groupIds
+      .map((groupId) => {
+        const reading = webAudio.groupLevel(groupId);
+        return reading ? { groupId, ...reading } : null;
+      })
+      .filter(
+        (entry): entry is { groupId: string; level: number; clipped: boolean } => entry !== null,
+      );
+    if (levels.length === 0) return;
+    postRuntimeMessage({ source: "hf-preview", type: "group-levels", levels });
+  };
+
   const finitePositiveDuration = (value: number | null | undefined): number =>
     typeof value === "number" && Number.isFinite(value) && value > 0 ? value : 0;
 
@@ -2897,6 +2916,9 @@ export function initSandboxRuntimeModular(): void {
       if (transportTickCount % 30 === 0) {
         bindMediaMetadataListeners();
       }
+      if (clock.isPlaying()) {
+        postGroupLevels();
+      }
 
       // Sync clock duration with the resolved timeline each tick (catches async
       // rebinds, live data-duration edits). Never shrink while playing — transient