Let a looped sample play out past its loop when the note ends #331

With "Mode" and "Loop" as two fields, One-shot plus Loop ran until
Stop was pressed: letting go of the key did nothing. The two fields
are now one, "Play", with four entries that each say what happens
while the note is held and when it is let go.

- Gate and One-shot play the sound once, as before. Loop repeats it
  and fades out with the note.
- "Loop, then release" is new: the sampler's sustain loop. Let go,
  it finishes the round it is in and plays what comes after "Loop
  end" to the end of the sound.
- A buffer source stops looping and plays on by itself. The pitch
  worklet gets a "leave" message. The phase vocoder then lets the
  round finish that its newest frame has read into, so nothing read
  already changes under the frames still sounding.
- Samples saved with "Mode" and "Loop" are turned into "Play" on
  load. Gate and One-shot stay, either with Loop becomes Loop, since
  playing until stopped is gone.
- The keys of the Instrument tab stay half lit while a note let go
  is still heard. Esc on the keys stops everything, like Stop.
- Striking a ringing key again plays a new note beside it rather
  than stopping the old one, so a drum can be hit quickly. This
  differs from what was proposed in the issue discussion.

Tests:
- instrument + pitch-player + sound-library + node-catalog:
  477 tests, green (12 new).
- e2e 34 (melektaus-atx): ALL PASSED, 45 checks (+1). Let go after
  a loop of 0.05 s, the note played out for 0.31 s on the audio
  clock, the key half lit meanwhile.
- Counter-check: a release that does not leave the loop fails the
  2 new player tests.

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

src/viewport/SoundLibrary.ts

@@ -50,6 +50,8 @@ interface Source {
   fade: GainNode | null;
   /** Ends with the note (#331): a release fades it out, where everything else plays on. */
   gate: boolean;
+  /** Plays on past its loop to the end of the sound; null where a release leaves it as it is. */
+  leave: (() => void) | null;
 }
 
 /**
@@ -222,14 +224,16 @@ export class SoundLibrary implements AudioSink {
   }
 
   /**
-   * The end of a note (#331): the ways that end with it fade out, the others play on, and the voice
-   * goes once they have all ended – or rings on first where it has echoes.
+   * The end of a note (#331): the ways that end with it fade out, a looping one that does not leaves
+   * its loop, the others play on, and the voice goes once they have all ended – or rings on first
+   * where it has echoes.
    */
   release(id: number): void {
     const waiting = this.pending.find((other) => other.id === id);
     if (waiting) waiting.released = true;
     const voice = this.voices.find((other) => other.id === id);
     if (!voice) return;
+    for (const source of voice.sources) if (!source.gate) source.leave?.();
     const context = this.audio();
     const gated = voice.sources.filter((source) => source.gate);
     for (const source of gated) if (source.fade) fadeOut(source.fade.gain, context.currentTime);
@@ -305,6 +309,11 @@ export class SoundLibrary implements AudioSink {
     return voice ? this.tailOf(voice.fx) : 0;
   }
 
+  /** Whether a voice is still heard – or waiting on its bytes to be. */
+  playing(id: number): boolean {
+    return this.voices.some((voice) => voice.id === id) || this.pending.some((other) => other.id === id);
+  }
+
   /** `letRing` stops the sound itself and lets its echoes fade as they would (#324). */
   stop(file: string, letRing = false): void {
     this.silence(file === '' ? this.voices : this.voices.filter((voice) => voice.file === file), letRing);
@@ -673,6 +682,7 @@ export class SoundLibrary implements AudioSink {
         },
         fade: null,
         gate,
+        leave: own && loop ? () => node.port.postMessage('leave') : null,
       });
       wiring.knobs.push({ param: node.parameters.get('pitch')!, target: pitch, even: 'value' });
       return node;
@@ -692,6 +702,13 @@ export class SoundLibrary implements AudioSink {
       },
       fade: null,
       gate,
+      // Web Audio plays a buffer that stops looping on to its end, past `loopEnd`.
+      leave:
+        own && loop
+          ? () => {
+              source.loop = false;
+            }
+          : null,
     });
     if (shifted) wiring.knobs.push({ param: null, target: pitch, even: 'value' });
     return source;