put_file hashed the source path, then copied the source again — two reads.
A source mutated in between would publish bytes whose digest != returned key,
breaking the content-addressed invariant. Now copy the source once into a
provisional staging file, derive the key from the SHA-256 of that staged file
(the exact bytes published), then dedup/atomic-rename. Guarantees
key == digest(published object) with a single source read.
Extends the acceptance test: re-hashes the stored object and asserts it equals
the returned key (and std.hash / shasum of the fixture), asserts cross-path
dedup (put_file and put_bytes of identical content share one object), and
asserts the staging temp is cleaned up on both the success and dedup paths.
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