test(calendar): add Vim + Neovim integration harnesses

Add headless harnesses covering the calendar-vim integration, wired into
CI alongside the existing keymap/config suites:

- test-calendar-vim.sh/.vim (+ -optout.vim): pure-VimL diary hooks
  (calendar_sign markers, calendar_action path resolution from wiki root,
  calendar window close), plugin/nuwiki.vim hook auto-wiring that
  overrides calendar-vim's defaults, and opt-out via g:nuwiki_use_calendar
  and g:nuwiki_no_calendar. Uses a stub calendar-vim on the runtimepath.
- test-calendar.sh/.lua: Neovim setup() wiring + window-close under the
  real window API, and opt-out via use_calendar = false.

Fixes surfaced by the suite:
- diary.vim: new diary entries set filetype 'vimwiki' (was the raw
  extension, e.g. 'wiki'); the stray FileType autocmd also blocked the
  calendar window from closing.
- init.lua: drop the redundant Neovim FileType autocmd (plugin/nuwiki.vim
  already wires both editors) and translate use_calendar=false into
  g:nuwiki_no_calendar so the opt-out actually disables wiring.

autoload/nuwiki/diary.vim

@@ -34,12 +34,10 @@ function! nuwiki#diary#calendar_action(day, month, year, week, dir) abort
   endif
 
   execute 'edit ' . fnameescape(l:file_path)
-  if !filereadable(l:file_path)
-    if l:c.ext[0] ==# '.'
-      execute 'setfiletype ' . l:c.ext[1:]
-    else
-      execute 'setfiletype ' . l:c.ext
-    endif
+  " A brand-new entry has no file on disk yet, so ftdetect may not have run;
+  " ensure the vimwiki filetype so syntax/maps are active immediately.
+  if &filetype !=# 'vimwiki'
+    setfiletype vimwiki
   endif
 
   " Close the calendar window now that the diary is open (keep at least one).