Merge origin/main so CI picks up the TabBar.context-menu fix (#20367).
- replaceReferenceLinks sliced the suffix and ran /^\s*(```|~~~)/ on every
blank line, so a reference definition after a long blank run made rich
mode quadratic again (~7s at 100k blank lines). Reuse the same cached
non-whitespace probe the rich encoder uses, keeping cross-line fence
semantics.
- Add a deadline-bounded regression test for the blank-run + reference
definition shape in raw-markdown-html.blank-run.test.ts.
Neil
e260406970