fix(multicursor): atom may cascade in another buffer #41624

Problem:
An atom queued in buffer A cascades on B's cursors if the mapping ends
in B ("nnoremap X x:bnext<CR>").

Solution:
Check the atom's origin buffer (`CmdAtom.origin.buf`).

Note: This does not preclude mappings etc from doing work in temporary
throwaway buffers, as long as they return to the origin buffer.

runtime/doc/repeat.txt

@@ -343,6 +343,7 @@ context), multicursor replays actions "semantically" (|CmdAtom|).
   line at cursor, not the entire folded contents). Motions step into folds
   instead of skipping over them. The primary cursor keeps the usual
   |fold-behavior|.
+- Cursors are per-buffer, and cascade only while their buffer is current.
 - CTRL-C interrupts the cascade.
 
 Example: place a cursor on every matching pattern: >