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/schemas/controller/links.py

@@ -180,6 +180,15 @@ class MarkerCreate(BaseModel):
         pattern=r"^(tx|rx)$",
         description="Direction filter: 'tx' = capture node sending only, 'rx' = capture node receiving only. Omitted or null = both directions.",
     )
+    capture_node_id: Optional[UUID] = Field(
+        None,
+        description=(
+            "Which endpoint's uBridge hosts this marker (the 'observer'). "
+            "tx/rx in `direction` are interpreted from this node's perspective. "
+            "Must be one of the link's two endpoints and a marker-capable type. "
+            "Omitted = server auto-picks (first started marker-capable endpoint)."
+        ),
+    )
 
 
 class MarkerDefinitionCreate(BaseModel):