Mesh observability: capture message and work movement for live cluster visualization

[snowdamiz]

Parent epic

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

Outcome

Capture data/message/work movement in a way that can power live visualizations without lying about what the runtime actually knows.

Acceptance criteria

• Cross-node movement is represented from real runtime data
• Flow semantics are clear enough for operator debugging
• The surface avoids fake animation divorced from real events