Mesh observability: emit structured runtime topology and actor lifecycle events

[snowdamiz]

Parent epic

• Mesh: built-in runtime observability canvas for nodes, actors, and message flow #15

Outcome

Expose a structured runtime event stream for node topology, actor lifecycle, and cluster state transitions.

Acceptance criteria

• Node join/leave/failure events are emitted
• Actor spawn/exit/restart events are emitted
• Event stream is stable enough for operator tooling and visualization consumers