marker: forward dir= from ubridge and add per-marker direction filter

Direction field in marker.match events
=======================================

Read ubridge dir=<tx|rx> from MARK signal datagrams and forward it
as a "dir" key in the marker.match notification event.  The field is
additive -- older ubridge builds omit it and the parser leaves it null,
so consumers fall back to undirected rendering with no version coupling.
Semantics are relative to the capture node (the signal's node=<id>):
tx = capture node is sending (ingressed device-side NIO), rx = it is
receiving (ingressed link-side NIO).

Per-marker direction filter (opt-in, server-side pipeline)
==========================================================

Add a direction field to MarkerCreate and MarkerDefinitionCreate
schemas ("tx" | "rx" | null).  Plumb it through the full pipeline:

  Schema -> controller (start_marker/update_marker, marker_entry,
  _markers_for_node, update_marker_definition sync)
  -> REST/MCP handlers -> compute _ubridge_add_marker_filter +
  IOU _ubridge_apply_markers -> bridge add_packet_filter dir <tx|rx>

When set, ubridge only fires the mark handler (signal + pcap) for
packets matching the chosen direction.  null (default/legacy) = both
directions -- zero behavioural change for existing markers.

Docs and tests
==============

- docs/features/marker-traffic-insight.md: signal format updated,
  new Direction section with NIO mapping, arrow mapping, and additive
  compatibility note.
- tests/compute/marker/test_marker_manager.py: 3 new parser tests
  (dir tx/rx/absent) plus existing test extended to assert dir=None.

13 files, +123/-22, 72 tests pass (zero breakage)

gns3server/compute/base_node.py

@@ -1084,7 +1084,7 @@ class BaseNode:
                 )
                 i += 1
 
-    async def _ubridge_add_marker_filter(self, bridge_name, name, bpf, pcap_path, tag=None, link_id=None):
+    async def _ubridge_add_marker_filter(self, bridge_name, name, bpf, pcap_path, tag=None, link_id=None, direction=None):
         """
         Attach a `mark` packet filter to a uBridge bridge for traffic insight.
 
@@ -1120,6 +1120,8 @@ class BaseNode:
         # so the link id is the only way to tell signals — and pcap files — apart.
         if link_id:
             cmd += f" link {link_id}"
+        if direction is not None:
+            cmd += f" dir {direction}"
         cmd += ' pcap "{path}"'.format(path=pcap_path)
         # Let BPF compile errors propagate — the marker is the user's intent, so a
         # bad expression must surface instead of being silently dropped.
@@ -1149,7 +1151,8 @@ class BaseNode:
                 markers_dir, f"{self._id}_{link_id}_{name}.pcap"
             )
             try:
-                await self._ubridge_add_marker_filter(bridge_name, name, bpf, pcap_path, tag, link_id)
+                await self._ubridge_add_marker_filter(bridge_name, name, bpf, pcap_path, tag, link_id,
+                                                     direction=spec.get("direction"))
             except UbridgeError as e:
                 # Swallow BPF compile errors (warn + skip) so a single bad
                 # expression can't break link creation / node restart — mirrors