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>

development/tests/test-keymaps-vim.vim

@@ -184,7 +184,7 @@ endfunction
 
 let s:mapping_surface = [
       \ ['<CR>', 'n'], ['<S-CR>', 'n'], ['<C-CR>', 'n'], ['<C-S-CR>', 'n'],
-      \ ['<M-CR>', 'n'],
+      \ ['<M-CR>', 'n'], ['<D-CR>', 'n'],
       \ ['<BS>', 'n'], ['<Tab>', 'n'], ['<S-Tab>', 'n'], ['+', 'nx'],
       \ ['<C-Space>', 'nx'], ['<C-@>', 'nx'], ['<Nul>', 'nx'],
       \ ['gnt', 'n'], ['gln', 'nx'], ['glp', 'nx'], ['glx', 'nx'],
@@ -201,7 +201,8 @@ let s:mapping_surface = [
       \ ['<A-Left>', 'n'], ['<A-Right>', 'n'],
       \ ['<Leader>ww', 'n'], ['<Leader>ws', 'n'], ['<Leader>wi', 'n'],
       \ ['<Leader>w<Leader>w', 'n'], ['<Leader>w<Leader>y', 'n'],
-      \ ['<Leader>w<Leader>t', 'n'], ['<Leader>w<Leader>i', 'n'],
+      \ ['<Leader>w<Leader>t', 'n'], ['<Leader>w<Leader>m', 'n'],
+      \ ['<Leader>w<Leader>i', 'n'],
       \ ['<Leader>wn', 'n'], ['<Leader>wd', 'n'], ['<Leader>wr', 'n'],
       \ ['<Leader>wh', 'n'], ['<Leader>whh', 'n'], ['<Leader>wha', 'n'],
       \ ['<Leader>wc', 'nx'], ['<C-Down>', 'n'], ['<C-Up>', 'n'],