Distinguish the two marker-policy fan-out paths in the log so we can tell
which is slow:
- apply_defs_to_new_link (project-open finalize): logs def count + link
count + elapsed, only when marker_definitions is non-empty.
- _marker_apply_concurrently (interactive create/update/delete def): logs
link count + elapsed per fan-out.
c877ed87a6