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>

development/tests/test-calendar-vim.vim

@@ -89,6 +89,24 @@ call s:record(winnr('$') == s:wins_before - 1, 'action.window_count_decremented'
 call s:record(&filetype ==# 'vimwiki', 'action.sets_filetype',
       \ 'filetype=' . &filetype)
 
+" ===== calendar follows the current wiki (multi-wiki) =====
+" Two wikis, each with its own diary entry. track_wiki() (driven by the
+" BufEnter autocmd in real use) switches which wiki the calendar reads.
+let s:wA = tempname() | call mkdir(s:wA . '/diary', 'p')
+let s:wB = tempname() | call mkdir(s:wB . '/diary', 'p')
+call writefile(['a'], s:wA . '/diary/2026-06-02.wiki')
+call writefile(['b'], s:wB . '/diary/2026-06-10.wiki')
+let g:nuwiki_wikis = [{'root': s:wA}, {'root': s:wB}]
+
+call nuwiki#diary#track_wiki(s:wA . '/index.wiki')
+call s:record(nuwiki#diary#calendar_sign(2, 6, 2026) ==# '*', 'track.wikiA_own_entry', '')
+call s:record(nuwiki#diary#calendar_sign(10, 6, 2026) ==# '', 'track.wikiA_not_wikiB_entry', '')
+
+call nuwiki#diary#track_wiki(s:wB . '/index.wiki')
+call s:record(nuwiki#diary#calendar_sign(10, 6, 2026) ==# '*', 'track.wikiB_own_entry', '')
+call s:record(nuwiki#diary#calendar_sign(2, 6, 2026) ==# '', 'track.wikiB_not_wikiA_entry', '')
+unlet g:nuwiki_wikis
+
 " ===== Wrap up =====
 
 call add(s:results, '')