Assert VPN fabric shard heartbeat

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

@@ -708,6 +708,7 @@ func TestHeartbeatPayloadIncludesMeshEndpointReport(t *testing.T) {
 		MeshProductionForwardingEnabled:  true,
 		MeshFabricSessionEnabled:         true,
 		VPNFabricSessionTransportEnabled: true,
+		VPNFabricSessionStreamShards:     6,
 		VPNFabricQUICMaxStreamsPerConn:   24,
 		MeshQUICFabricEnabled:            true,
 		MeshQUICFabricListenAddr:         ":19443",
@@ -759,11 +760,15 @@ func TestHeartbeatPayloadIncludesMeshEndpointReport(t *testing.T) {
 	if report, ok := payload.Metadata["vpn_fabric_session_transport_report"].(map[string]any); !ok ||
 		report["packet_payload"] != "rap.vpn_packet_batch.fabric.v1" ||
 		report["transport"] != "fabric_session_binary_frames" ||
+		report["stream_shards_per_class"] != 6 ||
 		report["peer_sessions"] == nil {
 		t.Fatalf("vpn fabric session report missing: %+v", payload.Metadata)
 	} else if report["quic_sessions"] == nil || report["quic_max_streams_per_conn"] != 24 {
 		t.Fatalf("vpn fabric quic session report missing: %+v", report)
 	}
+	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_endpoint_health_feedback"] != true {
 		t.Fatalf("vpn fabric endpoint health capability missing: %+v", payload.Capabilities)
 	}

docs/architecture/DISTRIBUTED_FABRIC_NODE_PROTOCOL_PLAN.md

@@ -408,6 +408,8 @@ The smoke report also exposes the mixed-batch frame fanout so regressions show
 up as a concrete fanout drop, not just a failed boolean.
 Batch fanout is bounded by configured stream shards, so a large batch with many
 flows cannot explode into unbounded fabric frames.
+Heartbeat tests assert the advertised VPN fabric stream-shard count and
+capability, keeping control-plane diagnostics aligned with runtime behavior.
 Fabric-session packet transport snapshots now report packets per stream plus
 last/max batch fanout, making real multi-site load distribution measurable from
 gateway status.