A source is now one source_content row and one file named by its
SourceKey, the SHA-256 of its host, path and query, made as the variant
key is. A store writes the file, then upserts the row, and removes the
file when the row cannot be written; an eviction deletes the row, then
the file, with no lock. source_metadata, the JSON files under
cache/metadata/, MetadataStorage, the content lock, the eviction
transaction and its test hook are gone. Two source URLs serving the same
bytes store them twice. Reconciliation still drops a source row whose
file is missing, as TestReconciliationReadsAPageAtATime counts those
reads. The tests of the removed sharing, lock and hook go with them.
Model: opus-5-5
