onFrameEncoded property

Optional callback fired immediately after each non-dropped frame is encoded, with the same Vp8FrameStats surfaced on EncodedVp8Frame.stats. Useful for streaming telemetry sinks (logger, metrics exporter, ABR driver) that prefer push to polling lastFrameStats. Not fired for dropped frames -- use onFrameDropped instead. Exceptions raised by the callback propagate out of encodeFrame. Mutable at runtime via setOnFrameEncoded.