feat: implement traffic insight marker backend (ubridge mark filter)

Add compute-side marker subsystem that receives ubridge UDP MARK signals
and forwards them as project-scoped notifications to the web UI for
real-time traffic coloring. Matched packets are always saved to per-link
pcaps for future replay.

Key components:
- gns3server/compute/marker/: MarkerManager (singleton, UDP listener +
  O(1) registry keyed by (node_id, filter_name)) + MarkerListener
  (DatagramProtocol parsing MARK lines per ubridge integration contract)
- gns3server/compute/base_node.py: marker sink/node config at ubridge
  startup; shared _ubridge_add_marker_filter / _ubridge_delete_marker_filter
- Per-node start_marker/stop_marker: VPCS (VPCS-{id}), QEMU
  (QEMU-{id}-{adapter}), Docker (bridge{adapter})
- Compute REST /markers/start + /markers/stop (vpcs/qemu/docker route files)
- Controller Link._markers state + UDPLink.start_marker/stop_marker/
  update_marker (mirror capture pattern: BPF validation, _choose_capture_side,
  node.post forwarding, topology persistence)
- Controller REST GET/POST/DELETE/PUT /v3/projects/{p}/links/{l}/markers
- Config: marker_listen_host / marker_listen_port in ServerSettings
- Signal routing: creation-time registry O(1) lookup, no node-table scan;
  project-scoped WS stream (not global); event payload always carries
  project_id for frontend scoping

Tests: 14 unit tests (registry, listener parsing, UDP round-trip);
562 existing tests pass with zero regressions.

gns3server/controller/link.py

@@ -88,6 +88,7 @@ class Link:
         self._link_type = "ethernet"
         self._suspended = False
         self._filters = {}
+        self._markers = {}
         self._link_style = {}
         self._wireshark = False
         self._show_filters_icon = True
@@ -99,6 +100,13 @@ class Link:
         """
         return self._filters
 
+    @property
+    def markers(self):
+        """
+        Get the traffic insight markers dict: name → {bpf, tag, enabled}
+        """
+        return self._markers
+
     @property
     def show_filters_icon(self):
         """
@@ -298,6 +306,27 @@ class Link:
 
         raise NotImplementedError
 
+    async def start_marker(self, name, bpf, tag=None):
+        """
+        Attach a traffic-insight marker to this link (base — UDPLink overrides).
+        """
+        raise NotImplementedError
+
+    async def stop_marker(self, name):
+        """
+        Remove a traffic-insight marker from this link (base — UDPLink overrides).
+        """
+        raise NotImplementedError
+
+    async def update_marker(self, name, bpf=None, tag=None, enabled=None):
+        """
+        Update an existing marker's BPF, tag, or enabled flag.
+
+        A BPF change is a delete+re-add on the ubridge side so the pcap is
+        flushed and the new filter takes effect.
+        """
+        raise NotImplementedError
+
     async def start_capture(self, data_link_type="DLT_EN10MB", capture_file_name=None, wireshark=False, jwt_token=None):
         """
         Start capture on the link
@@ -571,6 +600,7 @@ class Link:
                 "nodes": res,
                 "link_id": self._id,
                 "filters": self._filters,
+                "markers": self._markers,
                 "link_style": self._link_style,
                 "suspend": self._suspended,
                 "show_filters_icon": getattr(self, '_show_filters_icon', True),
@@ -585,6 +615,7 @@ class Link:
             "capture_compute_id": self.capture_compute_id,
             "link_type": self._link_type,
             "filters": self._filters,
+            "markers": self._markers,
             "suspend": self._suspended,
             "link_style": self._link_style,
             "wireshark": self._wireshark,