marker: serial-link (WAN) support via data_link_type -> uBridge linktype

Markers now work on serial links (Cisco HDLC / PPP / Frame Relay / ATM), not just Ethernet. A marker carries a data_link_type (default DLT_EN10MB); at the uBridge boundary it becomes the 'mark ... linktype <dlt>' keyword so the BPF compiles and the pcap is written with the matching link-layer.

- MarkerCreate / MarkerDefinitionCreate gain data_link_type (default DLT_EN10MB). Per-link it is create-only; definitions are updatable (a change re-fans-out).
- base_node._marker_linktype() normalizes the GNS3 DLT name (strip DLT_, uppercase, None for EN10MB). Single source is SerialPort.data_link_types, so Cisco PPP -> PPP_SERIAL (50), matching the capture path -- no second mapping table.
- _ubridge_add_marker_filter (generic) and the IOU marker loop append 'linktype <dlt>'.
- Definition fan-out branches on link_type in inherit_marker: Ethernet is always EN10MB; a serial link uses the definition's WAN encapsulation, or is SKIPPED when none was chosen (an EN10MB pcap on serial is undecodable). One definition covers a mixed topology.
- MCP marker_definition exposes data_link_type (None = not forwarded).
- No uBridge rebuild on a data_link_type change -- only that one marker's filter is swapped (delete + re-add), mirroring a BPF change; reset_packet_filters preserves sibling mark filters.

Requires the uBridge build with 'mark ... linktype' support.

gns3server/controller/udp_link.py

@@ -57,15 +57,18 @@ class UDPLink(Link):
 
     def _markers_for_node(self, node):
         """
-        Marker specs (name -> {bpf, tag, link_id, direction, enabled}) for the
-        markers whose capture side is ``node``. Routed by capture_node_id so a
-        marker only rides the NIO of the node whose uBridge will host it. A
-        disabled marker is included (installed then turned ``off`` at uBridge,
-        not dropped) so the UI can toggle it instantly without an NIO rebuild.
+        Marker specs (name -> {bpf, tag, link_id, direction, data_link_type,
+        enabled}) for the markers whose capture side is ``node``. Routed by
+        capture_node_id so a marker only rides the NIO of the node whose uBridge
+        will host it. A disabled marker is included (installed then turned
+        ``off`` at uBridge, not dropped) so the UI can toggle it instantly
+        without an NIO rebuild.
         """
         return {
             name: {"bpf": m["bpf"], "tag": m.get("tag"), "link_id": self._id,
-                   "direction": m.get("direction"), "enabled": m.get("enabled", True)}
+                   "direction": m.get("direction"),
+                   "data_link_type": m.get("data_link_type", "DLT_EN10MB"),
+                   "enabled": m.get("enabled", True)}
             for name, m in self._markers.items()
             if m.get("capture_node_id") == node.id
         }
@@ -350,7 +353,7 @@ class UDPLink(Link):
         # explicitly deletes a marker via the REST API, and a marker is torn
         # down automatically only when its link is deleted.
 
-    async def start_marker(self, name, bpf, tag=None, direction=None, capture_node_id=None, color=None, highlight_duration=None, enabled=True, inherited_from=None):
+    async def start_marker(self, name, bpf, tag=None, direction=None, data_link_type="DLT_EN10MB", capture_node_id=None, color=None, highlight_duration=None, enabled=True, inherited_from=None):
         """
         Attach a traffic-insight marker to this link.
 
@@ -401,6 +404,7 @@ class UDPLink(Link):
             "highlight_duration": highlight_duration,
             "capture_node_id": marker_side["node"].id,
             "direction": direction,
+            "data_link_type": data_link_type,
         }
         if inherited_from:
             marker_entry["inherited_from"] = inherited_from