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/runtime/systems/behaviorContext.ts

@@ -166,6 +166,10 @@ export function behaviorContext(
     },
     playSound: (assetId, volume, loop, detached, fx, fxParams) =>
       playSound(run, assetId, volume, loop, detached ? '' : entity.id, fx, fxParams),
+    soundLength: (assetId) => {
+      const file = soundFileOf(project, assetId);
+      return file ? run.audio.length(file) : 0;
+    },
     // An id nothing resolves stops nothing, rather than silencing the whole run by accident;
     // only the empty id means "everything", which is what the node promises.
     stopSound: (assetId) => {