Discard buffered frames when seeking while paused

[nnmarcoo]

Fixes #162

Seeking while a StreamingSound is paused doesn't discard the audio the decode scheduler already buffered ahead of the playhead, so frames from the old position play before any frame from the new one.

This implements the on_sync approach you suggested in the issue: seek_to and seek_by on the handle now also notify the StreamingSound, which drains the frame ringbuffer when playback isn't advancing. The scheduler can't drain it itself, since only the consumer side can pop.

Tests

• seek_while_paused_discards_stale_frames reproduces the issue and fails on main with the original symptom.
• seek_while_playing_keeps_buffered_frames covers the case where draining would be wrong. Removing the is_advancing guard makes it fail with silence where audio should be.

Note

• handle.position() briefly reports the pre-seek position after seeking, until the scheduler pushes frames from the new position. Position is derived from the index of frames in the ringbuffer rather than from the seek itself, so this predates this change and happens on main too. Mentioning it since it's adjacent, but it looked like a separate minor issue.

Let me know if you'd like a changelog entry. I wasn't sure whether those get written at release time.

[tesselode]

looks good, thanks!

i'll take care of the changelog