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/compute/vpcs/vpcs_vm.py

@@ -512,6 +512,42 @@ class VPCSVM(BaseNode):
             )
         )
 
+    async def start_marker(self, port_number, name, bpf, pcap_path, tag=None):
+        """
+        Attach a traffic-insight ``mark`` filter to this node's uBridge bridge.
+        On BPF match uBridge emits a MARK signal and appends the packet to the pcap.
+
+        :param port_number: port number (kept for API symmetry; VPCS has a single bridge)
+        :param name: stable filter name — pcap identity and echoed in MARK signals
+        :param bpf: libpcap BPF expression
+        :param pcap_path: absolute path ubridge appends matched packets to
+        :param tag: optional correlation id echoed in MARK signals
+        """
+
+        if self.ubridge:
+            await self._ubridge_add_marker_filter(f"VPCS-{self._id}", name, bpf, pcap_path, tag)
+        log.info(
+            "VPCS '{name}' [{id}]: starting marker '{marker}' on port {port_number}".format(
+                name=self.name, id=self.id, marker=name, port_number=port_number
+            )
+        )
+
+    async def stop_marker(self, port_number, name):
+        """
+        Remove a traffic-insight ``mark`` filter from this node's uBridge bridge.
+
+        :param port_number: port number (kept for API symmetry)
+        :param name: filter name previously passed to start_marker
+        """
+
+        if self.ubridge:
+            await self._ubridge_delete_marker_filter(f"VPCS-{self._id}", name)
+        log.info(
+            "VPCS '{name}' [{id}]: stopping marker '{marker}' on port {port_number}".format(
+                name=self.name, id=self.id, marker=name, port_number=port_number
+            )
+        )
+
     def _build_command(self):
         """
         Command to start the VPCS process.