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.lua

@@ -206,7 +206,7 @@ vim.defer_fn(function()
   local mapping_surface = {
     -- Links
     { '<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' },
     -- Lists
     { '<C-Space>', 'nx' }, { '<C-@>', 'nx' }, { '<Nul>', 'nx' },
@@ -227,7 +227,8 @@ vim.defer_fn(function()
     -- Wiki / diary / export
     { '<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' },