test: add Phase 5 integration tests, benchmarks, and bufpool tests
- Full integration test: two nodes on localhost exercising identity
creation, handshake, encrypted message exchange, UEPS packet routing
via dispatcher, and graceful shutdown
- Benchmarks: identity key generation, ECDH shared secret derivation,
KD-tree peer scoring (10/100/1000 peers), message serialisation,
SMSG encrypt/decrypt, challenge-response, UEPS marshal/unmarshal
- bufpool.go tests: buffer reuse verification, oversized buffer
discard, concurrent access, MarshalJSON correctness and safety
- Coverage: node/ 72.1% -> 87.5%, ueps/ 88.5% -> 93.1%
Co-Authored-By: Charon <developers@lethean.io>