feat(pipeline): transcribe guitar from guitar+piano+other stems

Demucs-6s scatters guitar energy across its guitar, piano and other stems.
Synthetic t1 median guitar F1 rises from 0.39-0.60 to 0.72-0.87; the real
'Last Night on Earth' reference goes 0.390 -> 0.403.

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

src/pipeline/run.ts

@@ -40,6 +40,19 @@ export function copyFingering(timed: TimedNote[], assigned: ScoreNote[], grid: G
   });
 }
 
+/** Sample-wise sum of equally shaped audio buffers. */
+export function sumAudio(...as: Audio[]): Audio {
+  const first = as[0]!;
+  return {
+    sampleRate: first.sampleRate,
+    channels: first.channels.map((c, k) => {
+      const o = new Float32Array(c.length);
+      for (const a of as) { const x = a.channels[k]!; for (let i = 0; i < o.length; i++) o[i]! += x[i]!; }
+      return o;
+    }),
+  };
+}
+
 export async function createPipeline(
   models: { demucs: Uint8Array; basicPitch: Uint8Array },
 ): Promise<(input: PipelineInput, flags: OracleFlags) => Promise<PipelineResult>> {
@@ -63,9 +76,10 @@ export async function createPipeline(
       }
     } else {
       const s = await separate(input.mix);
-      warnings.push('demucs "other" stem unused');
       const mk = (name: string, role: Role, audio: Audio): Job => ({ name, role, audio, truthNotes: null });
-      jobs.push(mk("Guitar", "guitar", s.guitar), mk("Bass", "bass", s.bass), mk("Keys", "keys", s.piano),
+      // Guitar is transcribed from guitar + piano + other: Demucs-6s scatters guitar energy across all three
+      // (synthetic t1 median guitar F1 0.39–0.60 → 0.72–0.87; real "Last Night on Earth" 0.390 → 0.403).
+      jobs.push(mk("Guitar", "guitar", sumAudio(s.guitar, s.piano, s.other)), mk("Bass", "bass", s.bass), mk("Keys", "keys", s.piano),
         mk("Vocals", "vocals", s.vocals), mk("Drums", "drums", s.drums));
     }