Code-review MAJOR MA2. The previous `store_blob(root, data: bytes)` signature
forced the entire payload into RAM before the 10 MB cap was checked — a
hostile-large output blob could OOM the worker before the limit even fired.
New signature: `store_blob(root, stream, *, max_bytes=10_485_760)`. The
implementation:
- reads from `stream` in 64 KB chunks;
- updates the sha256 + writes to `<root>/.tmp-<pid>-<rand>.gz` incrementally;
- raises `BlobTooLarge(max_bytes)` as soon as the running total crosses the
cap, then unlinks the partial temp file via `contextlib.suppress`;
- atomic-renames the temp file to the CAS path `<aa>/<bb>/<sha256>.gz` once
the stream finishes;
- sets `0o750` on the directory and `0o640` on the file with explicit
`os.chmod` (does not rely on the process umask).
Updated unit tests cover: BlobTooLarge enforcement (with temp-file cleanup),
multi-chunk happy path (1.5 MB payload exercising the 64 KB loop), and
`max_bytes <= 0` validation.
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