Closes the gap where LockCachingAudioSource wrote files to disk but
never told AudioCacheManager about them — meaning evict() couldn't
reclaim stream-cached files when usage exceeded the cap, only
explicitly-pinned downloads.
Wire just_audio's bufferedPositionStream as the "download complete"
signal: when bufferedPosition reaches duration (with 200ms slack for
header bytes), look up the on-disk file at the LockCaching path,
read its size, and insert an audio_cache_index row via the new
AudioCacheManager.registerStreamCache(). Source defaults to
incidental so stream-cached tracks are first to be evicted under
pressure.
Dedupe via _streamCacheRegistered Set so we don't hit drift on every
~200ms buffered-position emit. Cache the application cache dir path
on first use for the same reason.
Eviction now sees the full set of files on disk; usageBytes() (which
already walks the dir) and evict() (which reads the index) are
finally consistent for stream-cached tracks. Pinned tracks keep
their existing manual-download flow unchanged.
The Storage card has been showing "0 B" for users who only stream
(never explicitly pin or download an album). usageBytes() summed
SUM(size_bytes) from audio_cache_index — but the streaming path
through audio_handler writes files via LockCachingAudioSource without
ever inserting an index row, so the index undercounts (often to
zero) for normal use.
Walk the cache directory instead. Catches everything on disk:
manually pinned tracks (registered in the index), stream-cached
tracks (LockCaching), partial downloads. Falls back gracefully when
a file is racing against concurrent writes / deletes.
Eviction still operates on the index (it needs the source/recency
metadata to pick eviction order). Stream-cached files aren't subject
to eviction today — separate problem; addressed when we wire a
download-complete hook from LockCaching back into the index.
bvandeusen