Implement Core Cryptography Types

[pvandervelde]

Context

• Specification: docs/spec/modules/cryptography.md
• Location: src/crypto/
• Purpose: Types for encrypted messages and key management
• Dependencies: src/error.rs for crypto-specific errors
• Security: Must implement Debug carefully to avoid logging secrets
• Assertions: docs/spec/assertions.md fix(api): address API consistency and usability issues across public surface #24-37

Subtasks

• 19.1 Implement EncryptedMessage type with ciphertext, nonce, auth_tag, metadata
• 19.2 Implement EncryptionMetadata with algorithm and key_id fields
• 19.3 Implement custom Debug for EncryptedMessage (redact ciphertext)
• 19.4 Add encryption detection constants (MAGIC_MARKER "QRE1", VERSION)
• 19.5 Implement serialization/deserialization for wire format
• 19.6 Add unit tests for type construction and serialization

Phase

Phase 5: Cryptography Module Implementation (Task 19.0)