docs: add simulation controller SDK spec, replace spatial topology

The mesh is the communication fabric, not the simulation engine.
SimController pattern: external controller drives tick loop, computes
visibility, sends observations to peers, collects actions.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/vision-20260407.md

@@ -69,10 +69,19 @@ Per-mesh compute sandboxes. Peers request: `execute_code(lang: "python", code: "
 
 **Effort:** 2-3 days (E2B), 1-2 weeks (self-hosted).
 
-### Spatial topology (proximity-based visibility)
-Extend visibility with `(x, y)` coordinates and visibility radius. Peers only see others within range. Combined with sim clock, enables spatial simulations (customer walks into store zone, sees sales reps).
+### Simulation controller SDK (`@claudemesh/sim`)
+Replaces the "spatial topology" item. Instead of baking simulation features into the broker, build a framework on top of the mesh.
 
-**Effort:** 1 day.
+- **SimController class** — connects as a peer, drives tick loop, computes visibility, sends observations, collects actions
+- **World state** — entity system, zone graph, state namespacing (`sim:domain:key`)
+- **Visibility modes** — zone-based, radius, custom function (bring your own physics)
+- **Multi-controller** — federated domains (floor, inventory, queue) with orchestrator merge
+- **Dashboard integration** — reads world state from mesh, human input flows back via messages
+- **The mesh stays simple** — messaging + state. All simulation logic lives in the controller.
+
+Full spec: [`simulation-sdk-spec.md`](./simulation-sdk-spec.md)
+
+**Effort:** 5-6 days.
 
 ### Semantic peer search
 `search_peers(query, filters?)` — multi-field matching across names, groups, roles, summaries, profile capabilities, skills. Ranked results. For meshes with 50+ peers.