Add src/library/content.ts: a ContentCache keyed by fileID under
cacheDirectory (flat originals/ and thumbnails/, 0700/0600), fetching through
the request pools (#45) and the streaming decrypt / atomic writer (#40) so
present-means-complete. One shared pool set serves both this cache and the
ML-data fetch. Photo.original and thumbnail return {path,bytes}, skipped when
present; lib.thumbnails.ensure drives the thumbnail pool with priority, dedup,
and abort.
Integrity rests on the streaming decrypt (every chunk authenticated, renamed
in only on TAG_FINAL) plus a non-empty check. The stored-hash / size compare
is deferred (tracked in #68): the only in-repo hash fixture is a placeholder,
and the stored size is the encrypted object size, not the decrypted length.
Judgement call: three thumbnail priorities map onto two tiers.
Model: opus-4-8
sneak