feat(rpc): implement system.handshake, system.ping and system.shutdown (#7)

[ZhuchkaTriplesix]

Summary of Changes

• Implemented robust system.handshake validator returning protocol version, driver info, and comprehensive capabilities list (< 3s requirement satisfied).
• Implemented immediate system.ping handler replying "pong" ().
• Implemented system.shutdown handler returning {"ok": true} and initiating graceful process termination ().
• Upgraded src/main.rs dispatcher loop: fast-track life-cycle queries directly on the reader loop so that SandboxWatchdog heartbeats (system.ping) are never blocked even if heavy analytical queries (db.query) run concurrently on Tokio worker threads.
• Added 3 new unit tests in rpc::handlers::system ().

Closes #7