Add property-based tests for crypto operations

[careck]

Security Review Recommendation

Source: Krillnotes Security Review v1.0.1 (April 2026)

Description

The security review recommends adding fuzzing or property-based tests (quickcheck/proptest) for cryptographic edge cases, including:

• Nonce reuse detection
• Key collision scenarios
• Malformed input handling
• HLC ordering invariants
• Ed25519 signature round-trip stability

Recommendation

Add proptest or quickcheck as dev-dependency and write property-based tests for the core crypto primitives.

Acceptance Criteria

• proptest or quickcheck added as dev-dependency
• Property tests for at least: attachment encrypt/decrypt round-trip, signature sign/verify round-trip, HLC ordering
• Tests for nonce reuse, key collision, and malformed input scenarios