Problem: Argument type mismatches are excluded from EmmyLua checks.

Solution: Correct runtime annotations and calls, document local checker
workarounds, and enable the diagnostic.

AI-assisted

Problem:
Confirming cmdwin with a UTF-8 character containing 0x80 does not complete the
command.

Solution:
Escape K_SPECIAL bytes while feeding the cmdwin input after confirmation.

Problem:
Unreliable ui2 test:

    FAILED   …/ui/messages2_spec.lua @ 277: messages2 multiline messages and pager
    …/ui/messages2_spec.lua:277: Row 1 did not match.
    Expected:
      ...
      |*{1::}echo "foo" | echo "bar\nbaz\n"->repeat(&lines)      |
      |*{1::}messages                                            |
      |*{1::}^                                                    |
      ...
    Actual:
      ...
      |*{9:vim.schedule callback: ...ork/neovim/neovim/runt [+7]}|

Solution:
ui2 is doing contortions to handle the old cmdwin behavior; stop doing
that, it's no longer necessary since b2bf7bcfb151.

Problem:
Opening cmdwin from a 'scrollbind' window makes cmdwin scrollbinded.
Legacy cmdwin set 'noscrollbind'.

Solution:
Set 'noscrollbind'.

- Replace newlines in the current cmdline with NULs when opening cmdwin,
  and do the reverse when putting a cmdwin line back into the cmdline.
- Escape control characters with Ctrl-V when feeding cmdline.

Problem:
- If cmdwin window is split, ENTER in one does not close the others.
- If cmdwin is put into a different tabpage via <c-w>T, it stops working
  (ENTER does not execute the cmd).

Solution:
- Close the buffer instead of the window.
- In the WinClosed handler, skip `M._cleanup()` unless this is the last
  cmdwin window.

Problem: manual closing cmdwin blocks some events.

Solution: use `nested=true` in buffer-local `WinClosed` event.

Problem:
Legacy cmdwin set 'buflisted', but new one doesn't.
https://github.com/neovim/neovim/issues/40431#issuecomment-4811593353

'buflisted' useful for:
- "bufferline" style tablines, they usually show only listed buffers.
- some automatic actions can be conditioned on whether a buffer is
  listed; 'buflisted' signals that the buffer is "important enough".

Solution:
Set 'buflisted'.