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>

README.md

@@ -483,7 +483,7 @@ via `mappings.<group> = false` in Neovim, or
 | `<CR>` | Smart follow — inside `[[…]]` jumps; on a bare word wraps as `[[word]]` (second `<CR>` follows) |
 | `<S-CR>` | Follow in horizontal split |
 | `<C-CR>` | Follow in vertical split |
-| `<C-S-CR>` | Follow in a tab, reusing an existing tab if the file is already open |
+| `<C-S-CR>` / `<D-CR>` | Follow in a tab, reusing an existing tab if the file is already open (`<D-CR>` is the macOS Cmd alias) |
 | `<M-CR>` | Add the link target to the buffer list (`:badd`, no jump) |
 | `<BS>` | Jump back (`<C-o>`) |
 | `<Tab>` / `<S-Tab>` | Next / previous wikilink on or after the cursor |