probeClip abstract method
Probes clip source for its ClipMetadata (fps, frame count, size) and
caches it, so the render's clip pre-pass can plan which source frames to
extract before calling preResolveClip.
Idempotent: a probed source is served from cache. Throws a
FluvieRenderException when the source cannot be probed.
Implementation
Future<ClipMetadata> probeClip(MediaSource source);