feat(config): add planner.emergencyPressurePa for MGC rendezvous emergency bypass

Documented, defaults to null (inert). When set, the MGC pre-empts an in-flight
rendezvous lock and re-plans immediately if the resolved header pressure reaches
this canonical-Pa threshold. Mechanism is wired + tested; never fires until a
real value is configured.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/configs/machineGroupControl.json

@@ -149,6 +149,13 @@
           "type": "boolean",
           "description": "If true, every dispatch is routed through the rendezvous planner regardless of control strategy: per-pump moves are delayed so all pumps reach their setpoint at the same wall-clock instant t* = max(eta_i). If false, all flowmovement commands fire immediately and each pump ramps at its own speed (legacy behaviour)."
         }
+      },
+      "emergencyPressurePa": {
+        "default": null,
+        "rules": {
+          "type": "number",
+          "description": "Safety threshold (canonical Pa) for the rendezvous emergency bypass. While a rendezvous is in flight new setpoints are locked out and queued sequentially; if the resolved header pressure reaches this value the lock is pre-empted and the group re-plans immediately. Null/unset (the default) leaves the bypass mechanism wired but INERT — it never fires until a real threshold is configured."
+        }
       }
     },
     "mode": {