fix(extmarks): undo-redo of a mark explicitly moved during an edit #41252

Problem:
A mark moved by nvim_buf_set_extmark() during an edit is misplaced by
undo and redo. Only splices ("edits") are recorded, and replaying them
reproduces the shifts they caused, never the explicit set: the mark ends
up wherever the text pushed it.

Solution:
When an open undo block moves an existing mark, record both positions.
Undo restores the pre-set position, redo re-applies the set.

Partially reverts 18334a4a0cd9 ; ExtmarkSavePos.row/col were unused
because nothing recorded an explicit move, but now `extmark_set()` does.

0 files changed, 0 insertions, 0 deletions