Advertise VPN flow pressure capability

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

@@ -3273,6 +3273,7 @@ func heartbeatPayload(cfg config.Config, identity state.Identity, meshState *syn
 			payload.Capabilities["vpn_fabric_session_transport"] = true
 			payload.Capabilities["vpn_packet_batch_binary_frames"] = true
 			payload.Capabilities["vpn_fabric_session_stream_shards"] = true
+			payload.Capabilities["vpn_fabric_flow_pressure"] = true
 			if meshState != nil && meshState.VPNFabricEndpointObservations != nil {
 				payload.Metadata["vpn_fabric_endpoint_health_report"] = meshState.VPNFabricEndpointObservations.Report(observedAt, maxVPNFabricEndpointHealthReportEntries)
 			} else {

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

@@ -781,6 +781,9 @@ func TestHeartbeatPayloadIncludesMeshEndpointReport(t *testing.T) {
 	if payload.Capabilities["vpn_fabric_session_stream_shards"] != true {
 		t.Fatalf("vpn fabric stream shard capability missing: %+v", payload.Capabilities)
 	}
+	if payload.Capabilities["vpn_fabric_flow_pressure"] != true {
+		t.Fatalf("vpn fabric flow pressure capability missing: %+v", payload.Capabilities)
+	}
 	if payload.Capabilities["vpn_fabric_endpoint_health_feedback"] != true {
 		t.Fatalf("vpn fabric endpoint health capability missing: %+v", payload.Capabilities)
 	}

docs/architecture/DISTRIBUTED_FABRIC_NODE_PROTOCOL_PLAN.md

@@ -472,6 +472,8 @@ reasons.
 Heartbeat VPN fabric transport reports now include a compact
 `flow_pressure` summary with level, score, reasons, bulk pressure, route
 recovery timing, reason counts, and recommended per-class windows.
+Nodes advertise the `vpn_fabric_flow_pressure` capability when that heartbeat
+summary is available.
 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.