fix(client): small mapping/command-attribute parity gaps

Clean, fully-wired parity fixes (skipping the accept-but-ignore ones):

- <D-CR> (macOS Cmd+Return) → follow_link_drop, alongside <C-S-CR>, in both
  clients. Upstream binds both to VimwikiTabDropLink.
- plugin/nuwiki.vim global map block: fix the same diary collision the
  buffer-local maps had — <Leader>w<Leader>t now opens today in a new tab and
  <Leader>w<Leader>m (tomorrow) is added.
- Neovim TabIndex: add -count to NuwikiTabIndex and switch both
  Vimwiki/NuwikiTabIndex from vim.v.count (always 0 in command context) to
  <count>, so a count is actually honored.
- Converge VimwikiTableMoveColumn{Left,Right}: the Vim-branch commands now call
  the table_move_column_left/right aliases, matching the Neovim branch.
- VimwikiColorize/NuwikiColorize: -nargs=1 → -nargs=* (all four defs) for
  upstream parity; a bare :VimwikiColorize now prompts for the colour.

Docs (README + doc/nuwiki.txt) updated; mapping surfaces gain <D-CR> and
<Leader>w<Leader>m in both harnesses. vim 265+18, nvim 272, all green.

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

doc/nuwiki.txt

@@ -436,7 +436,7 @@ Links (normal mode) ~
   <S-CR>     Follow in a horizontal split.
   <C-CR>     Follow in a vertical split.
   <C-S-CR>   Follow in a tab, reusing an existing tab if the file is
-             already open.
+             already open. `<D-CR>` (macOS Cmd) is an alias.
   <M-CR>     Add the link target to the buffer list (`:badd`, no jump).
   <BS>       Jump back (`<C-o>`).
   <Tab>      Next wikilink on or after the cursor.