Three layers of integrity:
- Hash chain (SHA-256 chained) catches silent edits
- HMAC ratchet rotates keys every N entries with HKDF-style derivation
and zeroizes old key bytes from memory
- Merkle batch sealing builds tree over content_hashes, signs root
with Ed25519 (FFI to libcrypto EVP_PKEY_ED25519 directly since stdlib
lacks the high-level OpenSSL::PKey wrapper)
AuditLog.append is mutex-guarded for serial writes; verify_chain detects
both single-row tampering and structural inconsistency. BatchSealer's
pack_message format (BE start_seq || BE end_seq || merkle_root) is shared
between signer and verifier so external auditors can validate offline.
fe5e9cd07b