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/NodeContext.ts

@@ -250,6 +250,11 @@ export interface NodeContext {
     fx: string,
     fxParams: FxParams,
   ): number;
+  /**
+   * How many seconds one run of that sound asset lasts, so a change can span it; 0 where it does not
+   * exist or has not been loaded yet.
+   */
+  soundLength(assetId: string): number;
   /** Silences every voice of that sound; `''` silences everything this run started. */
   stopSound(assetId: string): void;
   /**