When seeking, the underlying decoder position was updated but the
BufferedStreamer's ring buffer still contained up to 2 seconds of
pre-seek audio data. The speaker would drain this stale buffer
before playing audio from the new position, causing an audible
delay where the old position's audio continued playing.
Add a Flush() method to BufferedStreamer that resets the ring buffer
pointers, and call it in seekLocked() immediately after a successful
seek. This ensures the speaker starts playing from the seeked
position without any stale audio artifact.