At-Rest Integrity: Verify-on-Read, Checksums, and GC
CAS keys make verify-on-read free: central verification, x-amz-checksum echo, tombstone GC, and corruption surfacing.
Checksum
S3 Integrity Checksums: x-amz-checksum-*
x-amz-checksum-CRC32/SHA256 echo on PUT; integrity surfaced to clients so dirty bytes never masquerade as clean. Teaches: client-verifiable integrity, Bolt zero-allocation on read path, Sentinel honesty guarantee.