[G2] Topology-changed broadcast for live visualization

[ammar-s847]

The ConnectomeView in hebb_app must reflect programmatic neuron/synapse creation without reopening the folder.

MVP: engine emits a topology-changed signal when nodes/edges are added/removed.

Acceptance:

• add/remove neuron + synapse emit a topology event on an existing broadcast channel
• event carries enough to incrementally update (ids + kind) or signals a refresh
• documented so hebb_app [F3] can subscribe

Dependencies: [G1] (shared ingest path).

Nice-to-haves: debounce/coalesce bursts from bulk seeds into one event.