fix: dataset layout data/<tier>/<seed>, mix tests, cli hardening

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

tests/audio/mix.test.ts

@@ -0,0 +1,25 @@
+import { test, expect } from "bun:test";
+import { mixStemsScaled } from "../../src/audio/mix.ts";
+import type { Audio } from "../../src/core/types.ts";
+
+test("mixStemsScaled: scaled stems sum to mix; scale is the normalisation factor", () => {
+  const mk = (n: number, amp: number): Audio => {
+    const x = Float32Array.from({ length: n }, (_, i) => amp * Math.sin(i / 7));
+    return { sampleRate: 44100, channels: [x, x.slice()] };
+  };
+  const a = mk(1000, 0.9), b = mk(700, 0.8);
+  const { mix, scaled, scale } = mixStemsScaled([{ audio: a, gainDb: 0 }, { audio: b, gainDb: -3 }]);
+  // unnormalised peak
+  const gb = Math.pow(10, -3 / 20);
+  let peak = 0;
+  for (let i = 0; i < 1000; i++) peak = Math.max(peak, Math.abs(a.channels[0]![i]! + (i < 700 ? b.channels[0]![i]! * gb : 0)));
+  expect(peak).toBeGreaterThan(1);
+  expect(Math.abs(scale - Math.pow(10, -1 / 20) / peak)).toBeLessThan(1e-6);
+  expect(scaled.length).toBe(2);
+  let maxDiff = 0;
+  for (let c = 0; c < 2; c++) for (let i = 0; i < 1000; i++) {
+    const s = scaled[0]!.channels[c]![i]! + scaled[1]!.channels[c]![i]!;
+    maxDiff = Math.max(maxDiff, Math.abs(s - mix.channels[c]![i]!));
+  }
+  expect(maxDiff).toBeLessThan(1e-6);
+});

tests/bench/dataset.test.ts

@@ -35,3 +35,23 @@ test("buildSong writes all artefacts; loadSongDir reads them; stems sum to mix;
     await rm(root, { recursive: true, force: true });
   }
 }, 120_000);
+
+test("multi-track t1b: stems on disk sum to mix.wav", async () => {
+  const root = await mkdtemp(join(tmpdir(), "stsheets-ds-"));
+  try {
+    const dir = await buildSong("t1b", 1, root);
+    expect(dir).toBe(join(root, "t1b", "1"));
+    const { song, mix, stems } = await loadSongDir(dir);
+    expect(song.tracks.length).toBeGreaterThan(1);
+    expect(stems.size).toBe(song.tracks.length);
+    let maxDiff = 0;
+    for (let c = 0; c < mix.channels.length; c++) for (let i = 0; i < mix.channels[c]!.length; i++) {
+      let s = 0;
+      for (const a of stems.values()) s += a.channels[c]![i] ?? 0;
+      maxDiff = Math.max(maxDiff, Math.abs(s - mix.channels[c]![i]!));
+    }
+    expect(maxDiff).toBeLessThan(2e-3);
+  } finally {
+    await rm(root, { recursive: true, force: true });
+  }
+}, 120_000);