feat: stem rendering, fx and mixing

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

src/audio/mix.ts

@@ -0,0 +1,28 @@
+import type { Audio } from "../core/types.ts";
+
+const PEAK_TARGET = Math.pow(10, -1 / 20);
+
+/** Sum stems (with per-stem gain in dB), then peak-normalise to -1 dBFS. */
+export function mixStems(stems: { audio: Audio; gainDb: number }[]): Audio {
+  if (stems.length === 0) throw new Error("mixStems: no stems");
+  const sr = stems[0]!.audio.sampleRate;
+  const nch = Math.max(...stems.map((s) => s.audio.channels.length));
+  const n = Math.max(...stems.map((s) => s.audio.channels[0]?.length ?? 0));
+  const out = Array.from({ length: nch }, () => new Float32Array(n));
+  for (const { audio, gainDb } of stems) {
+    if (audio.sampleRate !== sr) throw new Error("mixStems: sample rate mismatch");
+    const g = Math.pow(10, gainDb / 20);
+    for (let c = 0; c < nch; c++) {
+      const src = audio.channels[c] ?? audio.channels[0]!;
+      const dst = out[c]!;
+      for (let i = 0; i < src.length; i++) dst[i]! += src[i]! * g;
+    }
+  }
+  let peak = 0;
+  for (const ch of out) for (let i = 0; i < n; i++) peak = Math.max(peak, Math.abs(ch[i]!));
+  if (peak > 0) {
+    const k = PEAK_TARGET / peak;
+    for (const ch of out) for (let i = 0; i < n; i++) ch[i]! *= k;
+  }
+  return { sampleRate: sr, channels: out };
+}