Interface TrackSampleSink

Where AnimationClip.sampleAll gets its scratch and sends its results.

Split in two so the clip can sample without allocating and without knowing anything about targets. The mixer implements it over arrays indexed the same way as AnimationClip.tracks: one PropertyBinding per track, resolved once (§16), and one scratch value per track, allocated once.

const sink: TrackSampleSink = {
  outFor: (index) => scratch[index],
  applySample: (index, _track, value) => { bindings[index].set(value); },
};
clip.sampleAll(localTime, sink);          // allocates nothing
Copy

The two-step shape exists because of the mutatesInPlace split in ./values.js: for a Vector3 track the sampled value is the scratch object handed out by outFor, while for a number track it is a fresh primitive that outFor's storage never saw. Passing the result back through applySample is the only way to serve both without branching per sample.