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>

lua/nuwiki/keymaps.lua

@@ -166,6 +166,8 @@ function M.attach(bufnr, mappings)
         { desc = 'nuwiki: follow link in vsplit' }, bufnr)
     map('n', '<C-S-CR>', cmd.follow_link_drop,
         { desc = 'nuwiki: follow link in tab (reuse existing)' }, bufnr)
+    map('n', '<D-CR>',  cmd.follow_link_drop,
+        { desc = 'nuwiki: follow link in tab (macOS Cmd alias of <C-S-CR>)' }, bufnr)
     map('n', '<M-CR>',  cmd.badd_link,
         { desc = 'nuwiki: add link target to buffer list' }, bufnr)
     map('n', '<BS>',    '<C-o>', { desc = 'nuwiki: go back', remap = true }, bufnr)