test(engine): give the real-ffmpeg audio suites a timeout Windows can meet

`Tests on windows-latest` failed on two of the seven cases in
audioMixer.grouping.test.ts -- both `Test timed out in 5000ms`, not an assertion.
Every case in that suite mixes with REAL ffmpeg, and vitest's default 5s per test
is not enough for that on a Windows runner: the same suite passes on macOS and
ubuntu, and the job's own "Install FFmpeg" step logged a download-failure warning
before falling back. A suite-level 60s covers all seven at once rather than
seven per-test arguments.

audioMixer.level.test.ts gets the same treatment. It has not failed yet, but it
is the same real-ffmpeg shape one spawn slower away from it.

Both files are this branch's, so this is the branch's own flake to fix; engine
has no existing per-test timeout convention to follow, hence the suite argument
plus a comment saying which platform forced it.

packages/engine/src/services/audioMixer.grouping.test.ts

@@ -103,7 +103,12 @@ const track = (id: string, end: number, volume = 1) => ({
   type: "audio" as const,
 });
 
-describe.skipIf(!HAS_FFMPEG)("mix level arithmetic", () => {
+// Every case here mixes with REAL ffmpeg, and vitest's default 5s per test is not
+// enough for that on a Windows runner — two cases timed out there while passing
+// everywhere else (PR #3363). The suite-level timeout covers all of them at once.
+describe.skipIf(!HAS_FFMPEG)(
+  "mix level arithmetic",
+  () => {
     afterEach(() => {
       for (const dir of tempDirs.splice(0)) rmSync(dir, { recursive: true, force: true });
     });
@@ -393,4 +398,6 @@ describe.skipIf(!HAS_FFMPEG)("mix level arithmetic", () => {
 
       expect(Math.abs(meanVolumeDb(groupedOut) - meanVolumeDb(refOut))).toBeLessThan(0.5);
     });
-});
+  },
+  60_000,
+);

packages/engine/src/services/audioMixer.level.test.ts

@@ -55,7 +55,12 @@ function firstAudibleSeconds(path: string): number {
   throw new Error(`No audible sample found in ${path}`);
 }
 
-describe.skipIf(!HAS_FFMPEG)("processCompositionAudio levels", () => {
+// Same real-ffmpeg exposure as audioMixer.grouping.test.ts, which timed out on a
+// Windows runner at vitest's default 5s. This one has not failed yet; it is one
+// spawn slower away from it.
+describe.skipIf(!HAS_FFMPEG)(
+  "processCompositionAudio levels",
+  () => {
     afterEach(() => {
       for (const dir of tempDirs.splice(0)) rmSync(dir, { recursive: true, force: true });
     });
@@ -161,4 +166,6 @@ describe.skipIf(!HAS_FFMPEG)("processCompositionAudio levels", () => {
       expect(result.success).toBe(true);
       expect(firstAudibleSeconds(outputPath)).toBeCloseTo(2, 2);
     });
-});
+  },
+  60_000,
+);