fix(core): route the media-element transport through the group bus

main added `scheduleMediaElementPlayback` -- a pitch-preserving HTMLMediaElement
transport -- while this branch was open, and the runtime tries it FIRST for
audio, falling back to the decoded-buffer path only when it returns null. The
rebase therefore left every grouped track bypassing the very bus this branch
exists to add: the new path connected its gain straight to master, while only
the fallback went through `resolveDestination`.

It now uses `resolveDestination` too, which is a no-op for an ungrouped element
(it returns master) and the group's input gain for a member.

Three `init.test.ts` cases spied on `decodeAudioElement` to assert WHICH audio
elements get scheduled. That path is now the fallback, so the spies read zero
through no fault of the behaviour under test — they move to
`scheduleMediaElementPlayback`, with a comment saying why, and keep their
original claims: a `data-hidden` clip is excluded under the `audio-track-mute`
canary, still scheduled without it, and a two-clip un-hide is one reschedule.

core's runtime suites pass (init 78, webAudioTransport 61).

packages/core/src/runtime/init.test.ts

@@ -1323,16 +1323,20 @@ describe("initSandboxRuntimeModular", () => {
     // playing in preview for anyone not enrolled.
     window.__hf?.setCanaries?.({ "audio-track-mute": true });
 
-    const decodeSpy = vi
-      .spyOn(WebAudioTransport.prototype, "decodeAudioElement")
+    // `scheduleMediaElementPlayback`, not `decodeAudioElement`: the media-element
+    // transport is the path the runtime tries FIRST for audio, and the decoded
+    // buffer is only its fallback. What is under test either way is which
+    // ELEMENTS get scheduled at all.
+    const scheduleSpy = vi
+      .spyOn(WebAudioTransport.prototype, "scheduleMediaElementPlayback")
       .mockResolvedValue(null);
 
     const player = window.__player;
     player?.play();
     player?.seek(0);
 
-    expect(decodeSpy).toHaveBeenCalledTimes(1);
-    expect(decodeSpy.mock.calls[0]?.[0]).toBe(audibleAudio);
+    expect(scheduleSpy).toHaveBeenCalledTimes(1);
+    expect(scheduleSpy.mock.calls[0]?.[0]).toBe(audibleAudio);
   });
 
   it("still schedules a data-hidden audio clip when the host has not opted in", () => {
@@ -1356,16 +1360,16 @@ describe("initSandboxRuntimeModular", () => {
     window.__timelines = { main: createMockTimeline(10) };
     initSandboxRuntimeModular();
 
-    const decodeSpy = vi
-      .spyOn(WebAudioTransport.prototype, "decodeAudioElement")
+    const scheduleSpy = vi
+      .spyOn(WebAudioTransport.prototype, "scheduleMediaElementPlayback")
       .mockResolvedValue(null);
 
     const player = window.__player;
     player?.play();
     player?.seek(0);
 
-    expect(decodeSpy).toHaveBeenCalledTimes(1);
-    expect(decodeSpy.mock.calls[0]?.[0]).toBe(hiddenAudio);
+    expect(scheduleSpy).toHaveBeenCalledTimes(1);
+    expect(scheduleSpy.mock.calls[0]?.[0]).toBe(hiddenAudio);
   });
 
   it("batches a mid-playback data-hidden toggle into exactly one Web Audio reschedule", () => {
@@ -1406,8 +1410,8 @@ describe("initSandboxRuntimeModular", () => {
     // toggles away from.
     player?.play();
 
-    const decodeSpy = vi
-      .spyOn(WebAudioTransport.prototype, "decodeAudioElement")
+    const scheduleSpy = vi
+      .spyOn(WebAudioTransport.prototype, "scheduleMediaElementPlayback")
       .mockResolvedValue(null);
     const generationSpy = vi.spyOn(WebAudioTransport.prototype, "startGeneration");
 
@@ -1419,7 +1423,7 @@ describe("initSandboxRuntimeModular", () => {
     player?.seek(1, { keepPlaying: true });
 
     expect(generationSpy).toHaveBeenCalledTimes(1);
-    expect(decodeSpy).toHaveBeenCalledTimes(2);
+    expect(scheduleSpy).toHaveBeenCalledTimes(2);
   });
 
   // Scheduling does NOT replace the active set: it bumps a generation, which

packages/core/src/runtime/webAudioTransport.ts

@@ -272,7 +272,14 @@ export class WebAudioTransport {
       const elapsed = compositionTime - compositionStart;
       const timing: AutomationTiming = { scheduledAt, elapsed, rate: safeRate };
       const fx = attachElementFxChain(this._ctx, el, sourceNode, gainNode, timing);
-      gainNode.connect(this._masterGain);
+      // The group bus, not master, for a member — same as the decoded-buffer
+      // path. This transport is the PRIMARY one for audio (the decode path is
+      // its fallback), so routing it at master would have left every grouped
+      // track bypassing the bus whose whole premise is that a group is one
+      // signal.
+      gainNode.connect(
+        this.resolveDestination(el, scheduledAt, compositionTime, safeRate) ?? this._masterGain,
+      );
       scheduleVolumeLane(el, gainNode, timing);
 
       this._rate = safeRate;