Take one sound at the Output of an instrument, mixed by a Mix #331

The Output summed any number of edges as layers. That made it the only
sound input of its kind, and it read as if a note could be several
voices. A note was always one voice, but the graph did not say so.
Now the Output takes one sound like every other sound input, and
several are put together with a Mix in front of it (decided by the
user after #331).

- The Output's input is no longer `many`. A second edge replaces the
  first, as at any data input.
- With only one drum sounding for a note, a Mix is its other side. A
  drum kit keeps the volume of each drum.
- An instrument saved with several edges into the Output keeps them
  but does not compile. The Output says "Takes one sound: put a Mix in
  front of it to play several". No saved project has one: the only
  instrument there (cbbf160bb7e8) has a single edge.
- An Output with nothing in it is a problem now, not silence.
- The compiler gathers only triggers from several edges. The player
  still takes a list of layers, which an instrument fills with one.

Tests:
- instrument.test.ts: drum kit through a Mix, a second edge replacing
  the first, an old Output with two edges, an empty Output (33 tests)
- Counter-check: `many` back on the Output turns 15 tests red, the
  new one about the second edge among them
- e2e 34 on melektaus-atx: ALL PASSED, 47 checks
- Gate on melektaus-atx: CHECK EXIT 0, 244 test files, 3789 tests

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

src/core/instrument/compile.ts

@@ -1,5 +1,5 @@
 import { graphEmitter } from '../graph/emitGraph';
-import { type DeclaredPort, type Graph, graphFingerprint, type GraphNode } from '../graph/Graph';
+import { type DeclaredPort, edgesInto, type Graph, graphFingerprint, type GraphNode } from '../graph/Graph';
 import type { GraphLookup } from '../graph/lookup';
 import { resolveDef, terminalNode } from '../graph/nested';
 import { inlineValue, type NodeDef } from '../graph/NodeRegistry';
@@ -29,6 +29,7 @@ import {
   INSTRUMENT_PARAMETERS,
   INSTRUMENT_SAMPLE,
   INSTRUMENT_SOUND_FX,
+  InstrumentOutput,
   isInstrumentNodeDef,
   loops,
   noteOutputKey,
@@ -73,7 +74,7 @@ export function instrumentSourcesOf(project: Project): InstrumentSources {
 }
 
 export interface InstrumentBuild {
-  /** What the note is made of, each played on its own and summed; empty is silence. */
+  /** What the note is made of: the one sound at the Output, or none for silence. */
   layers: readonly FxPlan[];
   /** Per node id, what keeps the graph from compiling. */
   problems: Record<string, string>;
@@ -82,8 +83,8 @@ export interface InstrumentBuild {
 }
 
 /**
- * What travels along an edge of an instrument: sounds (none where nothing plays for this note, more
- * than one only into the Output), a number, or whether a trigger is on for this note.
+ * What travels along an edge of an instrument: sounds (none where nothing plays for this note), a
+ * number, or whether a trigger is on for this note.
  */
 type Value = { plans: FxPlan[] } | { amount: FxAmount } | { on: boolean };
 
@@ -116,7 +117,6 @@ export function compileInstrument(graph: Graph, note: number, sources: Instrumen
     emit: (def, inputs, node) => emitNode(def, inputs, node),
     unfed: (node, port) => {
       if (port.type === 'number') return { amount: { value: asNumber(inlineValue(node, port)) } };
-      if (port.type === 'audio' && port.many) return SILENT;
       walk.problem(
         node,
         port.type === 'trigger'
@@ -139,10 +139,8 @@ export function compileInstrument(graph: Graph, note: number, sources: Instrumen
       );
       return null;
     },
-    gather: (_node, port, values) =>
-      port.type === 'trigger'
-        ? { on: values.some((v) => 'on' in v && v.on) }
-        : { plans: values.flatMap((v) => ('plans' in v ? v.plans : [])) },
+    // Only a trigger takes more than one edge: either one turns it on.
+    gather: (_node, _port, values) => ({ on: values.some((v) => 'on' in v && v.on) }),
   });
 
   function emitNode(def: Def, inputs: Record<string, Value>, node: GraphNode): Record<string, Value> {
@@ -214,7 +212,10 @@ export function compileInstrument(graph: Graph, note: number, sources: Instrumen
 
   const output = graph.nodes.find((n) => n.type === INSTRUMENT_OUTPUT);
   if (!output) return { layers: [], problems: walk.problems, reason: 'No Output node' };
-  const arrived = walk.input(output, { key: 'in', label: 'in', type: 'audio', many: true });
+  // Saved while the Output still took any number of sounds: only the first would be heard.
+  if (edgesInto(graph, output.id, 'in').length > 1)
+    walk.problem(output, 'Takes one sound: put a Mix in front of it to play several');
+  const arrived = walk.input(output, InstrumentOutput.inputs[0]!);
   const failed = walk.firstProblem();
   if (failed !== null || !arrived || !('plans' in arrived))
     return { layers: [], problems: walk.problems, reason: failed ?? 'Cannot compile' };

src/core/instrument/nodes.ts

@@ -234,8 +234,8 @@ export const InstrumentOutput: InstrumentNodeDef = {
   category: 'sound',
   instrument: true,
   description:
-    'What is heard of a note. It takes any number of sounds and plays them together – the layers of the instrument.',
-  inputs: [{ ...audioIn, many: true }],
+    'What is heard of a note. It takes one sound; to play several together, put a Mix in front of it.',
+  inputs: [audioIn],
   outputs: [],
   params: [],
 };