fix: tempo octave choice, start-of-music anchoring, mean-normalised combs, pure toMono

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

src/dsp/mono.ts

@@ -0,0 +1,13 @@
+// Pure channel downmix. Re-exported from src/audio/wav.ts for existing importers.
+import type { Audio } from "../core/types.ts";
+
+/** Average all channels into one mono signal. */
+export function toMono(a: Audio): Float32Array {
+  const n = a.channels[0]?.length ?? 0;
+  if (a.channels.length === 1) return a.channels[0]!.slice();
+  const out = new Float32Array(n);
+  for (const ch of a.channels) for (let i = 0; i < n; i++) out[i]! += ch[i]!;
+  const k = 1 / a.channels.length;
+  for (let i = 0; i < n; i++) out[i]! *= k;
+  return out;
+}