Hand out the length of a played sound from Play sound

Wired into the time of Set sound FX parameters, a change then spans
exactly one run of the clip, so a fade or a filter sweep ends with its
last sample.

- Play sound has a second output, length, in seconds. It is 0 where
  nothing was played or the sound has not loaded yet.
- The AudioSink gives the length of a file, the SoundLibrary takes it
  from the decoded buffer. A pitch the voice starts on keeps it (#312).
- A pitch changed while the voice plays still changes its length, so a
  pitch sweep over length does not end with the sound. A pitch that
  keeps the length in real time is #317.

Tests:
- soundfx, sounds, keep-on-scene-load, node-catalog, sound-library:
  462 tests green.
- Counter-check: with length always 0, the catalog case and the new
  soundfx case fail.

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

src/core/graph/nodes/action/PlaySound.ts

@@ -61,7 +61,9 @@ export const PlaySound: NodeDef = {
     'unless "detached" is on. "fx" sends it through a sound FX asset; left empty, it plays as it is. ' +
     '"fx params" takes values for its parameters from a Sound FX parameters node. "voice" is the ' +
     'number of this one play, for Set sound FX parameters to change it while it plays; 0 where ' +
-    'nothing was played.',
+    'nothing was played. "length" is how many seconds one run of the sound lasts, to hand to the ' +
+    '"time" of Set sound FX parameters so a change spans the sound; 0 where nothing was played or ' +
+    'the sound has not been loaded yet. A pitch changed while it plays makes it longer or shorter.',
   inputs: [
     exec(),
     soundPort(),
@@ -71,7 +73,7 @@ export const PlaySound: NodeDef = {
     soundFxPort(),
     { key: 'fxParams', label: 'fx params', type: 'fxparams' },
   ],
-  outputs: [exec(), num('voice')],
+  outputs: [exec(), num('voice'), num('length')],
   params: [],
   run: (ctx, inputs) => {
     const sound = asString(inputs.sound);
@@ -85,7 +87,8 @@ export const PlaySound: NodeDef = {
           fxParamsOf(inputs.fxParams),
         )
       : 0;
-    return { next: 'exec', outputs: { voice } };
+    const length = voice ? ctx.soundLength(sound) : 0;
+    return { next: 'exec', outputs: { voice, length } };
   },
 };