test: mutation testing for Nostr event handling

[mostronatorcoder]

Context

Follow-up from #618. Implement mutation testing for Nostr event handling — the communication layer between Mostro and its clients.

Scope

• src/nip59.rs or NIP-59 gift wrap handling
• src/nip33.rs or parameterized replaceable events
• Event validation, signature verification
• Message routing and action dispatching

Goal

• Run mutation testing on Nostr modules
• Kill surviving mutants
• Target: >70% mutation score

Priority Mutants to Kill

• Event kind validation
• Signature verification logic
• Gift wrap encryption/decryption
• Message deserialization and action routing
• Pubkey validation

Acceptance Criteria

• Baseline mutation report for Nostr modules
• Critical mutants in event validation killed
• Critical mutants in action routing killed
• Mutation score documented in PR

Ref: #618