fix(marker): ride markers on NIO so they survive node restart and project reload

Mirror the packet-filter lifecycle: marker specs now live on the NIO
(next to filters), ride in link_data from controller to compute on
every NIO create/update, and are reapplied by _ubridge_apply_markers
in add_ubridge_udp_connection (bridge creation / node restart) and
update_ubridge_udp_connection (NIO update — following the preceding
reset_packet_filters so markers survive filter changes).

Changes:
- NIO / NIOUDP: _markers property + asdict
- schemas/compute/nios.py: UDPNIO.markers field
- base_manager.create_nio: nio.markers from settings
- PUT /nio routes (vpcs/qemu/docker): nio.markers update
- base_node: _ubridge_apply_markers(bridge_name, nio) iterates
  nio.markers, computes pcap path, calls _ubridge_add_marker_filter
  + MarkerManager.register; called after _ubridge_apply_filters
- controller udp_link: _get_node_markers + _markers_for_node (route
  by capture_node_id); markers in create() and update() link_data
- /markers/start,stop endpoints: mirror spec onto nio.markers so
  the marker survives a subsequent node stop/start without a PUT
- tests: add markers field to NIO data expectations

This covers:
- Node restart: NIO persists, add_ubridge_udp_connection re-applies
- Filter update: reset wipes markers, _ubridge_apply_markers re-adds
- Project reload: create() carries markers in link_data → create_nio
- Immediate create: endpoint sets nio.markers immediately

gns3server/compute/nios/nio.py

@@ -30,6 +30,7 @@ class NIO:
         self._capturing = False
         self._suspended = False
         self._filters = {}
+        self._markers = {}
         self._pcap_output_file = ""
         self._pcap_data_link_type = ""
 
@@ -118,3 +119,24 @@ class NIO:
 
         assert isinstance(new_filters, dict)
         self._filters = new_filters
+
+    @property
+    def markers(self):
+        """
+        Returns the traffic-insight markers for this NIO.
+
+        :returns: markers (dictionary: name -> {bpf, tag, link_id})
+        """
+
+        return self._markers
+
+    @markers.setter
+    def markers(self, new_markers):
+        """
+        Set the traffic-insight markers for this NIO.
+
+        :param new_markers: markers (dictionary: name -> {bpf, tag, link_id})
+        """
+
+        assert isinstance(new_markers, dict)
+        self._markers = new_markers

gns3server/compute/nios/nio_udp.py

@@ -80,5 +80,6 @@ class NIOUDP(NIO):
             "rport": self._rport,
             "rhost": self._rhost,
             "suspend": self._suspended,
-            "filters": self._filters
+            "filters": self._filters,
+            "markers": self._markers
         }