Mesh observability: build operator canvas for nodes, actors, failures, and respawns

[snowdamiz]

Parent epic

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

Outcome

Build the visual operator canvas that renders node topology, actor placement, flow, and failure/recovery behavior.

Acceptance criteria

• Nodes are visible on a canvas
• Actors per node are visible
• Node death and actor respawn are visibly represented
• The canvas reflects real runtime state rather than a static mock