fix(extmarks): redo of a mark created during an edit #41324

Problem:
A mark created by `nvim_buf_set_extmark()` while an undo block is open
never comes back on redo.

Analysis:
Undo deletes the text it covers and collapses the range; redo replays
the splices, which re-insert the text but cannot re-expand the mark.
`extmark_set()` records a position only for a mark it moves, not for one
it creates.

Solution:
Record the created position for redo; undo leaves the mark to the splice
replay, since it did not exist before the edit. Each side of a paired
mark gets its own entry. Redo also revives a mark that undo invalidated,
else its position returns but its highlight does not.

0 files changed, 0 insertions, 0 deletions