Spread fleet replicas across extra machines and show message RTT percentiles

machines.json plus Add-machine UI place copies on the least-loaded host.
Monitor samples processing RTT and displays min, avg, p50, and p90 for
capacity planning. Remote hosts run src/agent.js.

docs/02-architecture/fleet.md

@@ -18,4 +18,17 @@ operator  --HTTP 127.0.0.1:3850-->  fleet control
 - **pause / off** — pause does not count toward `min`; keepFloor starts another tree-node. `stop` on a service disables it.
 - **tree-node floor** — `fleet.json` `tree-node.min` (default 3). Do not drop this without changing the spec; bulk-summary leaf queries need several bloom-filtered nodes.
 
+## Extra machines
+
+Define hosts in `packages/verae-fleet/machines.json` (or **Add machine** on the monitor). Each machine has `capacity` and `roles` it may run. The supervisor places new replicas on the **least-loaded** eligible host.
+
+- `kind: local` — spawn on the control plane.
+- `kind: agent` — HTTP to `http://host:agentPort` (`node src/agent.js` on that box). Do not publish NATS.
+
+Raising `tree-node.max` and adding machines increases bulk-summary lookup capacity.
+
+## Message RTT (planning)
+
+The monitor samples `POST /message` on each healthy replica and shows **min / avg / p50 / p90** milliseconds of message processing (service row, instance, and machine). Use p90 when sizing extra tree nodes.
+
 Zapier cloud is not spawned. NATS is monitored, not bound publicly.