Local blob store under src/store/, the first real consumer of std.hash.
Objects are addressed by lowercase-hex SHA-256: the digest is the storage
key and bytes live at <root>/objects/<sha256>.
- put_bytes / put_file compute the digest via std.hash, write to a
staging file, then atomically rename into objects/<sha256>. The rename
is the only step that publishes, so an interrupted/failed write never
leaves a torn object at the final path.
- Dedup: an already-published object short-circuits without re-staging.
- stage_write/stage_copy + publish expose the two phases for the test.
tests/store_content_addressed.sx asserts the storage key equals std.hash,
an independent `shasum -a 256`, and the pinned SHA-256("abc") vector;
that dedup stores one object and never rewrites it; that a staged write
is invisible until publish and a failed publish leaves no object; and
that put_file round-trips bytes. Gate: make build + make test both green.
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