feat(parity): P3 quick-wins — ShowVersion, gLH/gLL/gLR, Search/Align nargs

Closes four low-risk P3 parity items (gap doc updated):

- VimwikiShowVersion / NuwikiShowVersion: new global command (both clients)
  echoing g:nuwiki_version (0.1.0) + the host editor, via
  nuwiki#commands#show_version / commands.show_version.

- gLH / gLL / gLR: upstream's case-variant aliases of gLh/gLl/gLr (dedent /
  indent whole item, renumber all lists) added in both clients.

- VimwikiSearch / VWS: -nargs=1 -> -nargs=* (Vim + Neovim), so a multi-word
  search no longer raises E488 and an empty invocation reuses the last search
  pattern. (The lvimgrep-vs-vimwiki-engine difference stays open.)

- VimwikiTableAlignQ / AlignW / NuwikiTableAlign: bare -> -nargs=?, so
  `:VimwikiTableAlignQ 2` no longer raises E488 (optional column arg is
  accepted-and-ignored; the gqq-align vs gww-align-without-resize split
  stays open).

Docs (doc/nuwiki.txt + README) updated for the command and the aliases.
Tests: surface.{Vimwiki,Nuwiki}ShowVersion, map[n].gL{H,L,R} (both
harnesses), cmd.search_and_tablealign_nargs (vim). Neovim 291, Vim
282/18/21 pass.

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

autoload/nuwiki/commands.vim

@@ -168,6 +168,13 @@ endfunction
 " from the server, so the picker works from any buffer before a wiki file is
 " ever opened — opening the chosen index then auto-starts the LSP via the
 " FileType autocmd. Plain Vim has no `vim.ui.select`, so use `inputlist()`.
+" :VimwikiShowVersion / :NuwikiShowVersion — echo the nuwiki version and the
+" host editor, mirroring upstream's version banner.
+function! nuwiki#commands#show_version() abort
+  echo 'nuwiki: ' . get(g:, 'nuwiki_version', '?')
+  echo (has('nvim') ? 'Neovim' : 'Vim') . ': ' . v:version
+endfunction
+
 function! nuwiki#commands#wiki_ui_select() abort
   let l:wikis = nuwiki#commands#wiki_list()
   if empty(l:wikis)