Add Cryptography Module Tests and Documentation

[pvandervelde]

Context

• Specification: docs/spec/modules/cryptography.md section 9
• Location: src/crypto/*_tests.rs files
• Coverage: Unit tests, integration tests, property tests, security tests
• Documentation: Rustdoc with examples, security considerations
• Assertions: All assertions fix(api): address API consistency and usability issues across public surface #24-37

Subtasks

• 24.1 Add property tests with proptest (verify encryptdecrypt roundtrip)
• 24.2 Add security test: verify constant-time tag verification (Assertion ci(release-plz): add workflow_dispatch trigger to release-plz PR workflow #32)
• 24.3 Add multi-service integration test (sender receiver with shared KeyProvider)
• 24.4 Add observability test: verify metrics and logs for encrypted=true/false
• 24.5 Add comprehensive rustdoc to all public crypto types and traits
• 24.6 Add security warnings to documentation (never log keys, zero on drop)
• 24.7 Add usage examples to crypto module documentation
• 24.8 Run full test suite and verify all assertions pass

Phase

Phase 5: Cryptography Module Implementation (Task 24.0)