Captures from the stream's current audio track from now on, keeping the same context and worklet, so the chunks keep flowing to the same callback. A source node reads the track it was created with, so a stream whose track was replaced (a device switch) needs this. Does nothing after Stop.
Stops capture and releases the audio context / worklet resources.
Handle returned by CreatePcmMicCapture: a driver rebinds it when the microphone's track changes and stops it on teardown. Production wraps an
AudioContext+AudioWorkletNodepipeline; tests return a fake.