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/link.py

@@ -122,13 +122,27 @@ class Link:
         with a per-link private marker of the same name.  It carries an
         ``inherited_from`` back-reference that (a) guards against per-link
         edits and (b) lets the project sync changes to every copy at once.
+
+        The pcap link-layer follows the link type: Ethernet is always EN10MB.
+        A serial link needs the definition's WAN encapsulation (HDLC / PPP /
+        Frame Relay); if none was chosen the serial link is skipped — an EN10MB
+        pcap on a serial link is undecodable.
         """
 
+        def_data_link_type = marker_def.get("data_link_type", "DLT_EN10MB")
+        if self._link_type == "serial":
+            if def_data_link_type.upper() == "DLT_EN10MB":
+                return  # definition is Ethernet-only; skip this serial link
+            data_link_type = def_data_link_type
+        else:
+            data_link_type = "DLT_EN10MB"
+
         await self.start_marker(
             name=f"global-{def_name}",
             bpf=marker_def["bpf"],
             tag=marker_def.get("tag"),
             direction=marker_def.get("direction"),
+            data_link_type=data_link_type,
             color=marker_def.get("color"),
             highlight_duration=marker_def.get("highlight_duration"),
             enabled=not marker_def.get("paused", False),