The daemon only prefetched the next track after a gapped advance, so every
edge after a seamless hand-off (and every QConnect/API play) reached the
10 s gapless window cold and had to materialize the whole next file in
time. Whether it made it was file size vs bandwidth (#699 follow-up on a
1 GB Pi: gapless "sometimes, varies by album"; alternating pattern).
Add a `WarmSuccessor` driver action fired once per current track id on
the first tick that sees it, for every entry point, run as a driver-owned
task that never blocks the control loop; drop the awaited prefetch from
`advance_and_play`. For a successor that is still not cached at arm time,
append its initial CMAF buffer as an incremental stream
(`queue_gapless_streaming`, the desktop's hand-off) and keep the
whole-file byte fetch only as fallback.
Tests: 2 new driver tests (warm once per track for any entry, seamless
edge warms the new successor); qbz-app 269 and qbzd 298 pass.
cc
vicrodh
e90ea8cada