feat(calendar): diary into the current wiki, not always the first

The VimL calendar-vim callbacks (nuwiki#diary#calendar_action /
calendar_sign) hardcoded wiki 0, so opening the calendar while editing
ifood_wiki still created/marked diary entries under the first wiki. (The
LSP-driven diary commands already follow the current buffer via its URI;
this brings the calendar in line.)

Track the wiki of the active buffer (nuwiki#diary#track_wiki, driven by
FileType/BufEnter autocmds in plugin/nuwiki.vim) and resolve the calendar
diary path against it, falling back to the first wiki. Verified the
calendar marker/open switch between wikis as you move between their
buffers. test-calendar-vim gains 4 multi-wiki tracking checks (16).
All keymap/config/calendar harnesses green on both clients.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

autoload/nuwiki/diary.vim

@@ -3,6 +3,33 @@
 
 " ===== Calendar-Vim Integration =====
 
+" The wiki index (0-based) of the most recently entered wiki buffer. The
+" calendar shows / creates diary entries for this wiki, so opening the calendar
+" while editing `ifood_wiki` diaries into ifood_wiki — not always the first
+" wiki. Updated by the BufEnter autocmd in plugin/nuwiki.vim; defaults to 0.
+" (The LSP-driven diary commands already follow the current buffer via its URI;
+" this mirrors that for the VimL calendar-vim callbacks.)
+let s:current_wiki_idx = 0
+
+" Record the wiki that owns `file` as the current one. Called on entering a
+" wiki buffer. No-op when the file is under no configured wiki.
+function! nuwiki#diary#track_wiki(file) abort
+  let l:file = fnamemodify(a:file, ':p')
+  if empty(l:file)
+    return
+  endif
+  let l:n = 0
+  for l:w in nuwiki#commands#wiki_list()
+    let l:root = fnamemodify(expand(l:w.root), ':p')
+    if l:root[len(l:root) - 1:] !=# '/' | let l:root .= '/' | endif
+    if l:file[: len(l:root) - 1] ==# l:root
+      let s:current_wiki_idx = l:n
+      return
+    endif
+    let l:n += 1
+  endfor
+endfunction
+
 " Function: nuwiki#diary#calendar_action(day, month, year, week, dir)
 " Called by calendar-vim when user presses Enter on a date.
 " day/month/year are numeric integers; dir is 'V' for a vertical split (any
@@ -12,8 +39,8 @@ function! nuwiki#diary#calendar_action(day, month, year, week, dir) abort
   " Build YYYY-MM-DD from the parts passed by calendar-vim
   let l:date_str = printf('%04d-%02d-%02d', a:year, a:month, a:day)
 
-  " Get wiki configuration for the first wiki (index 0)
-  let l:c = s:wiki_cfg(0)
+  " Wiki of the buffer the calendar was opened from (falls back to the first).
+  let l:c = s:wiki_cfg(s:current_wiki_idx)
 
   " Construct file path: {root}/{diary_rel}/{YYYY-MM-DD}.{ext}
   let l:file_path = l:c.root . '/' . l:c.diary_rel . '/' . l:date_str . l:c.ext
@@ -62,8 +89,8 @@ function! nuwiki#diary#calendar_sign(day, month, year) abort
   " Build YYYY-MM-DD from the parts passed by calendar-vim
   let l:date_str = printf('%04d-%02d-%02d', a:year, a:month, a:day)
 
-  " Get wiki configuration for the first wiki (index 0)
-  let l:c = s:wiki_cfg(0)
+  " Wiki of the buffer the calendar was opened from (falls back to the first).
+  let l:c = s:wiki_cfg(s:current_wiki_idx)
 
   " Construct expected file path
   let l:file_path = l:c.root . '/' . l:c.diary_rel . '/' . l:date_str . l:c.ext