marker: drive uBridge enabled/pause/resume in real time

Wire gns3-server to uBridge's real-time marker controls (contract
../ubridge/doc/gns3server-integration.md §3.2), in three layers:

A. enabled reaches uBridge — _markers_for_node no longer drops disabled
   markers controller-side; it carries `enabled` in the NIO spec. apply
   installs every marker then issues `enable_packet_filter … off` for the
   disabled ones (base_node bridge / iou iol_bridge; old-ubridge errors
   downgrade to a warning so a toggle can't break link create).

B. instant per-filter toggle — apply records name→bridge so a new
   `_ubridge_set_marker_filter_state` can flip a running filter with
   `enable_packet_filter on|off` (iou overrides for iol_bridge + bay/unit).
   Each marker-capable node type gains PUT /markers/{name}; update_marker
   short-circuits to it when only `enabled` changes (no NIO rebuild, no pcap
   flush), falling back to reset+reapply if the route is unavailable.

C. global pause/resume — `_ubridge_marker_pause/resume` send `marker pause`
   / `marker resume` direct to the hypervisor (pause stops signal+pcap,
   resume instant, sink retained). Six node-type routes add POST
   /markers/pause|resume; project.pause_all/resume_all_markers fan out to
   each capture node (deduped, best-effort); REST exposes
   POST /projects/{id}/markers/pause|resume.

Toggling enabled and pause/resume are now both instant — only marker create
or a bpf change still go through reset+reapply.

gns3server/compute/iou/iou_vm.py

@@ -1321,9 +1321,25 @@ class IOUVM(BaseNode):
                     self.project.emit("log.warning", {"message": message})
                     continue
                 raise
+            if not spec.get("enabled", True):
+                try:
+                    await self._ubridge_send(f"iol_bridge enable_packet_filter {location} {name} off")
+                except UbridgeError as e:
+                    log.warning(f"Could not turn marker '{name}' off on {location}: {e}")
             manager.register(
                 str(self.project.id), self._id, name, link_id, tag
             )
+            # Record name -> location (bridge bay unit) for instant toggle.
+            self._marker_filter_bridges[name] = location
+
+    async def _ubridge_set_marker_filter_state(self, name, enabled):
+        """IOU override: toggle via ``iol_bridge enable_packet_filter <bridge> <bay> <unit> <name> on|off``."""
+
+        location = self._marker_filter_bridges.get(name)
+        if not location:
+            raise UbridgeError(f"Marker '{name}' is not installed on this node")
+        state = "on" if enabled else "off"
+        await self._ubridge_send(f"iol_bridge enable_packet_filter {location} {name} {state}")
 
     async def adapter_remove_nio_binding(self, adapter_number, port_number):
         """