marker: let callers pin the capture node via capture_node_id

A marker is single-sided — only the chosen capture node's uBridge installs the
mark filter — and dir=tx|rx is interpreted from that node's perspective. Until
now the observer was always auto-picked (_choose_marker_side), so dir=tx meant
"the auto-chosen endpoint is sending", which is unpredictable and makes the
direction filter hard to render meaningfully in the Web UI.

Add an optional create-only capture_node_id to MarkerCreate: when set, the
marker is pinned to that endpoint's uBridge (validated as a link endpoint and a
marker-capable type); when omitted, behavior is unchanged (auto-pick). The
chosen id is already echoed back as capture_node_id and in MARK signals, so the
UI can always render the observer regardless of who picked it.

capture_node_id is create-only (changing it would silently flip the meaning of
stored direction; recreate instead) and is not accepted on project-level
definitions — they are link-agnostic and have no endpoints, so inherited
markers keep auto-picking per link.

Plumbed through REST create_marker, the MCP link_marker tool, and base
Link.start_marker. update_marker does not forward it.

gns3server/api/routes/controller/links.py

@@ -466,6 +466,7 @@ async def create_marker(
         bpf=marker_data.bpf,
         tag=marker_data.tag,
         direction=marker_data.direction,
+        capture_node_id=marker_data.capture_node_id,
         color=marker_data.color,
         highlight_duration=marker_data.highlight_duration,
     )

gns3server/api/routes/mcp/__init__.py

@@ -1016,6 +1016,7 @@ async def link_marker(
     tag: Annotated[int | None, Field(description="Numeric tag for packet correlation")] = None,
     enabled: Annotated[bool | None, Field(description="Enable or disable the marker (for update action)")] = None,
     direction: Annotated[str | None, Field(description="Direction filter: 'tx' for capture node sending only, 'rx' for capture node receiving only (omit for both)")] = None,
+    capture_node_id: Annotated[str | None, Field(description="UUID of the endpoint whose uBridge hosts the marker (the observer; tx/rx are from its perspective). Must be a link endpoint and marker-capable. Omit to auto-pick.")] = None,
     color: Annotated[str | None, Field(description="Hex color for UI highlight, e.g. '#ff5722'")] = None,
     highlight_duration: Annotated[int | None, Field(description="UI highlight duration in milliseconds")] = None,
 ) -> list[dict[str, Any]]:
@@ -1034,7 +1035,7 @@ async def link_marker(
     and cannot be modified or deleted via this tool.
     """
     params = {"project_id": project_id, "link_id": link_id, "action": action}
-    for opt in ("bpf", "marker_name", "name", "tag", "enabled", "direction", "color", "highlight_duration"):
+    for opt in ("bpf", "marker_name", "name", "tag", "enabled", "direction", "capture_node_id", "color", "highlight_duration"):
         val = locals().get(opt)
         if val is not None:
             params[opt] = val

gns3server/api/routes/mcp/links.py

@@ -395,7 +395,7 @@ def link_marker_handler(params: dict[str, Any], gns3_ctx: dict[str, Any]) -> dic
         if not bpf:
             return {"error": "bpf is required for create action"}
         body: dict[str, Any] = {"bpf": bpf}
-        for opt in ("name", "tag", "direction", "color", "highlight_duration"):
+        for opt in ("name", "tag", "direction", "capture_node_id", "color", "highlight_duration"):
             if params.get(opt) is not None:
                 body[opt] = params[opt]
         return conn.http_call("post", base, json_data=body).json()