feat: add structured task supervision for graceful shutdown

[qj0r9j0vc2]

Summary

This PR implements structured task supervision for the CipherBFT node, resolving the issue of missing coordinated shutdown for background workers.

Changes

• New supervisor.rs module: Implements NodeSupervisor using tokio-util's TaskTracker and CancellationToken for:

  • spawn() - Regular task spawning with cancellation support
  • spawn_cancellable() - Tasks that handle their own cancellation
  • spawn_critical() - Tasks that trigger full shutdown on failure
  • shutdown() - Graceful shutdown with configurable timeout

• Refactored node.rs:

  • Added run_with_supervisor() method for external supervisor control
  • Peer-connector task now runs under supervisor
  • Event loop respects cancellation token with biased select

• Updated main.rs:

  • cmd_start() creates supervisor and sets up Ctrl+C signal handler
  • cmd_testnet_start() uses shared supervisor for all validators

Shutdown Order

1. Stop accepting new network connections (Primary shutdown)
2. Drain in-flight consensus rounds (via cancellation token)
3. Flush pending storage writes (supervisor waits for tasks)
4. Close database connections
5. Exit

Test Plan

• All 66 existing tests pass
• 3 new supervisor unit tests added and passing:
  • test_supervisor_spawn_and_shutdown
  • test_supervisor_cancellation
  • test_critical_task_failure_triggers_shutdown

Closes #56