feat(diary): -count wiki selector for diary-note + index commands (P3)

Closes the last command-cluster gap. Upstream's diary-note family
(:Vimwiki{Make,TabMake,MakeYesterday,MakeTomorrow}DiaryNote + VimwikiDiaryIndex,
and the Nuwiki* forms) carry -count=0 where the count selects the wiki number;
nuwiki's were bare. Implemented as a real end-to-end selector, reusing existing
infrastructure:

- Server (commands.rs): OptUriArg gained an optional `wiki` selector;
  resolve_diary_wiki(backend, uri, wiki) now prefers it (via the existing
  resolve_wiki_selector, which already handles a 0-indexed wiki number) over
  the buffer URI. Used by diary_open_relative + diary_open_index.

- Clients: s:diary_open / _diary_open (both) and diary_today/today_tab/
  yesterday/tomorrow/index take a count and send {wiki: count-1} when >0. All
  22 diary-note + DiaryIndex command defs across the 4 contexts are now
  -count=0 passing <count>. diary_next/prev ignore the count.

Tests: cmd.VimwikiMakeDiaryNote_has_count (test-keymaps.lua) +
cmd.Vimwiki{MakeDiaryNote,DiaryIndex}_accepts_count (test-keymaps-vim.vim).
Workspace test/clippy/fmt clean with CI flags; lua 284, vim 272/18/21.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

development/tests/test-keymaps.lua

@@ -906,6 +906,15 @@ vim.defer_fn(function()
       error(':NuwikiGenerateTags not defined')
     end
   end)
+  -- Diary-note family carries -count=0 (vimwiki's wiki selector), so
+  -- `:2VimwikiMakeDiaryNote` selects wiki #2 instead of raising E481.
+  tobj_case('cmd.VimwikiMakeDiaryNote_has_count', function()
+    local c = vim.api.nvim_buf_get_commands(0, {}).VimwikiMakeDiaryNote
+    if not c then error(':VimwikiMakeDiaryNote not defined') end
+    if c.count == nil or c.count == false then
+      error('VimwikiMakeDiaryNote missing -count: ' .. vim.inspect(c))
+    end
+  end)
   -- `:VimwikiNormalizeLink 1` (the arg is upstream's visual flag; the command
   -- is -nargs=? not -range, so no `'<,'>`) wraps the last visual selection.
   tobj_case('cmd.normalize_link_visual_wraps_selection', function()