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.
19 controller-layer tests covering start/stop/update_marker (storage,
inheritance guards + bypasses, partial-update preservation of render
hints), project-def CRUD (fan-out, sync, delete-cleanup regression),
apply_defs_to_new_link, persist_markers/asdict, and aggregation.
15 API-route tests covering per-link create/update/delete (201/200/204),
global-prefix rejection on create (409 regression), bad-format rejection
(422), PUT/DELETE-on-inherited guard (409 regression), project-def CRUD
endpoints, and the aggregation view.
All 34 tests pass when run as part of the full suite.
