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>

tests/helpers.ts

@@ -270,6 +270,8 @@ export function fakeContext(overrides: Partial<NodeContext> = {}): FakeContext {
     // The number of a voice is how many have been played: unique, and never 0.
     playSound: (assetId, volume, loop, detached, fx, fxParams) =>
       ctx.played.push({ assetId, volume, loop, detached, fx, fxParams }),
+    // Every sound lasts a quarter of a second, as if it had been loaded.
+    soundLength: () => 0.25,
     stopSound: (assetId) => ctx.silenced.push(assetId),
     tuned: [],
     tuneSound: (voice, fxParams, time, ease) => ctx.tuned.push({ voice, fxParams, time, ease }),