Implement framed envelope-encrypted representations and key rotation

[flyingrobots]

Outcome

Implement the authenticated encrypted-representation ADR with bounded framed encryption, multi-recipient envelope keys, exact verification, and representation-only key rotation.

Dependencies

• ADR: define authenticated encrypted representations and key capabilities #86 authenticated encrypted-representation ADR
• Implement precise verification reports and corruption refusal #20 precise verification reports
• [COOL IDEA™] Add bounded streaming write-through path for read→write and copy pipelines #72 bounded transfer semantics
• [BAD CODE™] In-memory reference staging still materializes all missing chunks before publish #74 bounded staging

Invariant impact

Keep must return the exact logical bytes named by BlobId only after the complete selected encrypted representation has satisfied its framing, AAD, authentication, layout, and reconstruction laws.

Acceptance criteria

• Encryption and decryption stream within explicit frame/count/byte bounds.
• A random DEK is generated per representation and wrapped independently for admitted recipients.
• Every frame binds canonical AAD and refuses reordering, duplication, omission, truncation, and cross-representation substitution.
• Recipient add/remove and KEK rotation do not re-encrypt payload frames or move BlobId.
• Rotation returns a receipt binding old and new representation/key-envelope coordinates without exposing key material.
• No plaintext is committed to caller output before the relevant authenticated boundary succeeds.
• Golden, mutation, corruption, property, fuzz, process-death, recovery, and public-contract tests pass.
• Benchmarks report throughput, peak memory, frame overhead, and key-rotation work.
• The crypto dependency and every enabled feature receive a written dependency review.

Non-goals

• External key-store administration.
• Deterministic encryption or plaintext-equality deduplication.
• Application naming or access-control policy.
• Treating key rotation as physical deletion.