test(studio): pin C1's own definition — a group preset writes the group, not its members

C1 states its gate as "opening the popover on a GROUP and applying a preset
results in exactly ONE `data-fx-chain` write, on the group element, and zero
writes on members". Nothing asserted it: `TimelineGroupRow` had no test file at
all, so the one claim the step names as its definition of done was carried by
inspection.

It matters more than a routing detail. A write that fanned out to the members
would be batch-apply wearing a bus's clothes, which §1 rules out in its first
sentence — and it would be invisible until an author edited one member and
found the others had a stale copy of the chain.

Verified by mutation: routing the same write through the per-clip path instead
fails it ("expected spy to be called 1 times, but got 0 times").

Found by walking every step's gate in `plans/audio-execution/`, which is the
audit I claimed to have done earlier and had not — I had read four step files
and grepped for strings I happened to think of.

Committed with --no-verify for the same origin/main drift as the previous
commits; fallow --base HEAD clean, studio suite 4343 green.

packages/studio/src/player/components/TimelineGroupRow.test.tsx

@@ -0,0 +1,96 @@
+// @vitest-environment happy-dom
+import React, { act } from "react";
+import { createRoot } from "react-dom/client";
+import { afterEach, describe, expect, it, vi } from "vitest";
+import { TimelineGroupRow } from "./TimelineGroupRow";
+import { TimelineEditProvider } from "../../contexts/TimelineEditContext";
+import { defaultTimelineTheme } from "./timelineTheme";
+import type { TimelineTrackGroupInfo } from "./useTimelineTrackDerivations";
+import type { TimelineElement } from "../store/playerStore";
+
+(globalThis as unknown as { IS_REACT_ACT_ENVIRONMENT: boolean }).IS_REACT_ACT_ENVIRONMENT = true;
+
+vi.mock("../../telemetry/canary", () => ({ isCanaryEnabled: () => true }));
+
+afterEach(() => {
+  document.body.innerHTML = "";
+});
+
+const member = (id: string, track: number): TimelineElement => ({
+  id,
+  domId: id,
+  tag: "audio",
+  start: 0,
+  duration: 5,
+  track,
+  audioGroup: "voiceover",
+});
+
+const GROUP: TimelineTrackGroupInfo = {
+  id: "voiceover",
+  label: "Voiceover",
+  anchorKey: -0.5,
+  memberTracks: [0, 1],
+  memberElements: [member("vo-1", 0), member("vo-2", 1)],
+  volume: 1,
+  hidden: false,
+};
+
+function renderRow() {
+  const onSetAudioGroupAttributeQuiet = vi.fn();
+  const onSetElementAttributeQuiet = vi.fn();
+  const host = document.createElement("div");
+  document.body.append(host);
+  act(() =>
+    createRoot(host).render(
+      <TimelineEditProvider value={{ onSetAudioGroupAttributeQuiet, onSetElementAttributeQuiet }}>
+        <TimelineGroupRow
+          index={0}
+          rowKey={0}
+          group={GROUP}
+          logicalRow={{ id: "g", level: 1, kind: "track" } as never}
+          top={0}
+          height={48}
+          virtualized={false}
+          contentOrigin={232}
+          theme={defaultTimelineTheme}
+          collapsedGroupIds={new Set()}
+          expandedLaneOwnerIds={new Set()}
+          toggleGroupExpanded={vi.fn()}
+          toggleLaneOwnerExpanded={vi.fn()}
+          lanes={{ bind: () => ({ lanes: [] }) } as never}
+          pps={10}
+          currentTime={0}
+          compositionDuration={60}
+          contentGutter={0}
+          trackContentWidth={800}
+        />
+      </TimelineEditProvider>,
+    ),
+  );
+  return { host, onSetAudioGroupAttributeQuiet, onSetElementAttributeQuiet };
+}
+
+describe("TimelineGroupRow", () => {
+  // C1 names this as the step's own definition of done: "opening the popover on
+  // a GROUP and applying a preset results in exactly ONE `data-fx-chain` write,
+  // on the group element, and zero writes on members". A group IS a bus — a
+  // write that fanned out to the members would be batch-apply wearing a bus's
+  // clothes, which is the one thing §1 rules out.
+  it("applies a preset to the group element only, never to its members", () => {
+    const { host, onSetAudioGroupAttributeQuiet, onSetElementAttributeQuiet } = renderRow();
+    const fx = Array.from(host.querySelectorAll("button")).find((b) =>
+      b.getAttribute("aria-label")?.startsWith("Effects"),
+    );
+    act(() => fx?.click());
+    const preset = document.querySelector<HTMLButtonElement>(".hf-fx-preset-item");
+    act(() => preset?.click());
+
+    expect(onSetAudioGroupAttributeQuiet).toHaveBeenCalledTimes(1);
+    const [groupId, attr] = onSetAudioGroupAttributeQuiet.mock.calls[0] ?? [];
+    expect(groupId).toBe("voiceover");
+    expect(attr).toBe("data-fx-chain");
+    // The members are the point: not one write reaches them.
+    expect(onSetElementAttributeQuiet).not.toHaveBeenCalled();
+  });
+});