The head, tail and content columns of the files table are BLOB NOT
NULL and hold the raw 32-byte SHA-256, half the size of the hex text,
in the table and in the files_signature index. The hash phase, the
records, report and trees carry each hash as a [sha256.Size]byte and
compare it with ==; nothing is hex-encoded. Reading a record whose
hash column is not 32 bytes is an error naming its path.
In the tests, testHash is a stand-in hash: a string's bytes padded
with zeros, so fixtures keep the sort order their strings give.
Judgement call: a hash of the wrong length is refused on reading,
not copied silently.
Model: opus-5-5
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