Addresses items raised by the multi-agent code review of the 0.9.9
HW accel + first-start work:
- EncoderProfile now carries DecodeHwAccel so the demuxer `-hwaccel`
flag and the encoder argv derive from a single resolved profile.
Adding a new backend can no longer leave the two switches out of
sync.
- VAAPI no longer passes `-hwaccel_output_format vaapi`. That option
pinned decoded frames to GPU memory, but the filter chain (scale,
format, setparams) runs on CPU and would fail with "impossible to
convert between formats". Frames now decode HW + flow on CPU; the
encoder uploads back to GPU. Pre-existing bug, never reported because
no one had VAAPI auto-detected in practice.
- readyMax field comment + name: documented that it's a COUNT
(segments ready), not an index. The semantics were correct but the
comment read "highest index" which made `idx < readyMax` look like
an off-by-one to reviewers.
- probe_cache background janitor: 5-minute sweeper that drops expired
entries even when no lookup retouches the key. Lookup-only eviction
was fine for small libraries but unbounded for users who browse and
abandon thousands of files within a TTL window. Lazy + sync.Once.
- probe_cache TTL eviction now re-checks under the write lock so a
concurrent re-insert isn't accidentally evicted.
- probe_cache size-change test now Chtimes the file back to its
original mtime so only `size` differs between store and lookup
keys — properly exercises the size-check path.
- New TestProbeCache_SweepDropsExpired covers the janitor sweep.
- CHANGELOG: backfilled missing compare links 0.6.4 → 0.9.9.
- Stale "line ~1119" reference in VideoToolbox comment dropped; the
bitrate block moved a few lines and the comment was already wrong.
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