- Add checksum.py module with SHA256 utilities and HashingStreamWrapper
- Add ChecksumMismatchError and VerifyingStreamWrapper for verification
- Add get_verified() and get_stream_verified() to storage layer
- Add verify query param and verify_mode (pre/stream) to download API
- Add checksum headers: X-Checksum-SHA256, Digest, ETag, X-Verified
- Add comprehensive unit tests for checksum utilities
- Add integration tests for download verification API
