Windowed candidate search breaks marker tracking for big insertions

When speeding up a line-tracking diff (keeping bookmarks or breakpoints on their lines across a whole-file reload) by searching only the N lines nearest an estimated target, check where the target comes from. If it is the marker's offset from the top of the unmatched gap, inserting more than N lines directly above an edited marker line pushes the real line out of the window, and the marker is lost. Inserting the same lines below works. Search near both the top-relative and the bottom-relative target, (gapEnd - (oldGapEnd - line)). It's cheap, and it keeps the speedup while restoring the old placement. A probe that inserts N+50 lines above and below an edited marked line catches it.