feat(editor): undo restores cursor position #41520

Problem:
Undo places the cursor wherever the cursor happened to sit at "save
time" (`uh_cursor` is sampled lazily on the first change).
Examples:
- `i` preserves, but `a` does not
- `diw`, `atest<Esc>`, `d^` abandon the original position
- `D`, `o` restore it (by accident).

Solution:
`composite` tracks the pending atom (and its `origin`) across frames.
A `stuffed` continuation frame inherits the `origin` + prepped redo.
Store `origin` info in the undo header, so undo can restore it.

- Not for a mid-command undo break (i_CTRL-G_u).
- Undoing a mapping restores where the mapping started (which
  technically may be different than where the "edit" started).

test/functional/lua/buffer_updates_spec.lua

@@ -167,6 +167,8 @@ describe('lua: nvim_buf_attach on_lines', function()
     tick = tick + 1
     check_events { { 'test2', 'lines', 1, tick, 1, 1, 2, 0 } }
 
+    -- Undo restored the cursor (|restore-undo-cursor|); pin it for the relative edits below.
+    api.nvim_win_set_cursor(0, { 5, 0 })
     feed('wix')
     tick = tick + 1
     check_events { { 'test2', 'lines', 1, tick, 4, 5, 5, 16 } }