Implement the store package with SHA-256 keyed blob storage using a
two-level directory layout (blobs/XX/YY/hash). Supports atomic writes
via temp file + rename, deduplication, and hash validation. Includes
comprehensive tests for round-trip, deduplication, existence checks,
deletion, and subdirectory creation.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
