Stabilize VPN flow pressure heartbeat

agents/rap-node-agent/cmd/rap-node-agent/main.go

@@ -3265,6 +3265,9 @@ func heartbeatPayload(cfg config.Config, identity state.Identity, meshState *syn
 			if meshState != nil && meshState.VPNFabricSessionDialStats != nil {
 				report["dial_stats"] = meshState.VPNFabricSessionDialStats.Report(observedAt)
 			}
+			report["flow_pressure"] = vpnFabricFlowPressureReport(vpnruntime.FabricFlowSchedulerSnapshot{
+				PressureLevel: "nominal",
+			})
 			if meshState != nil && meshState.VPNFabricIngress != nil {
 				ingressSnapshot := meshState.VPNFabricIngress.Snapshot(identity.ClusterID)
 				report["flow_pressure"] = vpnFabricFlowPressureReport(ingressSnapshot.FlowScheduler)

docs/architecture/DISTRIBUTED_FABRIC_NODE_PROTOCOL_PLAN.md

@@ -474,6 +474,8 @@ Heartbeat VPN fabric transport reports now include a compact
 recovery timing, reason counts, and recommended per-class windows.
 Nodes advertise the `vpn_fabric_flow_pressure` capability when that heartbeat
 summary is available.
+When the VPN fabric ingress runtime has not been initialized yet, the heartbeat
+still emits a nominal `flow_pressure` summary for schema stability.
 Endpoint ranking treats `capacity_limited` observations as a soft pressure
 penalty instead of a hard recent failure, enabling load spreading without
 marking the carrier unhealthy.