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/Audio.ts

@@ -28,6 +28,11 @@ export interface AudioSink {
    * where it is not given). 0 is a quick glide.
    */
   tune(voice: number, params: FxParams, time?: number, ease?: EaseShare): void;
+  /**
+   * How many seconds one run of that file lasts, the same with a pitch it starts on (#312); 0 while
+   * its bytes have not arrived.
+   */
+  length(file: string): number;
   /** Silences every voice of that file; `''` silences all of them. */
   stop(file: string): void;
   /** Silences every voice started by that owner – what destroying a behavior does. */
@@ -37,6 +42,7 @@ export interface AudioSink {
 export const SILENT_AUDIO: AudioSink = {
   play: () => 0,
   tune: () => {},
+  length: () => 0,
   stop: () => {},
   stopOwner: () => {},
 };

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) => {