fix(studio-server): decode the same audio stream the peak map probes

The channel probe read stream a:0 while the decode left stream choice to
ffmpeg, which picks a later stereo stream over a mono first one; every frame
was then read at the wrong width, doubling the bins and taking the other
stream's level. Both commands now select a:0, and the cache key moves to
peaks-v2 so maps built from the wrong stream are rebuilt.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: vanceingalls

packages/studio-server/src/helpers/peakMap.test.ts

@@ -66,7 +66,7 @@ describe("buildPeakMapCacheKey", () => {
     expect(a).toBe(buildPeakMapCacheKey("a/talk.mp4", { size: 10, mtimeMs: 1 }));
     expect(a).not.toBe(buildPeakMapCacheKey("a/talk.mp4", { size: 11, mtimeMs: 1 }));
     expect(a).not.toBe(buildPeakMapCacheKey("a/talk.mp4", { size: 10, mtimeMs: 2 }));
-    expect(a).toMatch(/^peaks-v1_a_talk\.mp4_10-1\.json$/);
+    expect(a).toMatch(/^peaks-v2_a_talk\.mp4_10-1\.json$/);
   });
 });
 
@@ -109,4 +109,41 @@ describe("decodePeakMap", () => {
       }
     },
   );
+
+  it.skipIf(!ffmpeg)(
+    "decodes the probed first audio stream when a louder stereo stream follows it",
+    async () => {
+      const dir = mkdtempSync(join(tmpdir(), "hf-peaks-"));
+      try {
+        const file = join(dir, "multi.mkv");
+        execFileSync(ffmpeg ?? "ffmpeg", [
+          "-v",
+          "error",
+          "-f",
+          "lavfi",
+          "-i",
+          "aevalsrc=0.1*sin(2*PI*440*t):s=48000:d=1",
+          "-f",
+          "lavfi",
+          "-i",
+          "aevalsrc=0.9*sin(2*PI*440*t)|0.9*sin(2*PI*440*t):s=48000:d=1",
+          "-map",
+          "0:a",
+          "-map",
+          "1:a",
+          "-c:a",
+          "pcm_f32le",
+          "-disposition:a",
+          "0",
+          file,
+        ]);
+        const peaks = await decodePeakMap(file);
+        expect(peaks.length).toBeGreaterThanOrEqual(19);
+        expect(peaks.length).toBeLessThanOrEqual(21);
+        expect(Math.max(...peaks)).toBeCloseTo(0.1, 2);
+      } finally {
+        rmSync(dir, { recursive: true, force: true });
+      }
+    },
+  );
 });

packages/studio-server/src/helpers/peakMap.ts

@@ -5,7 +5,7 @@ import { findFfBinary } from "@hyperframes/parsers/ff-binaries";
 export const PEAK_BIN_SECONDS = 0.05;
 const PEAK_SAMPLE_RATE = 48_000;
 const execFileAsync = promisify(execFile);
-const PEAK_MAP_CACHE_VERSION = "peaks-v1";
+const PEAK_MAP_CACHE_VERSION = "peaks-v2";
 
 /**
  * Running max-abs per bin over interleaved f32le at the source's own channel
@@ -92,6 +92,8 @@ export async function decodePeakMap(mediaPath: string): Promise<number[]> {
         "error",
         "-i",
         mediaPath,
+        "-map",
+        "0:a:0",
         "-vn",
         "-ar",
         String(PEAK_SAMPLE_RATE),