Once the doubling reached the RAM preview's `fit`, the held sound waited for
offsets 0..fit, but the prefetch only requests offsets below fit - 1, and
while the silent preview moves on its last requests are still rendering. The
sound then stayed silent almost to the end of the preview even after
rendering recovered.
The backoff now stops at half of the frames the prefetch requests, and never
below the wait before any stall. A new test caches only the frames playback
itself requests, with a RAM preview of `ahead + 2` frames.
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>