feat(client): conceal colour spans down to their coloured text

A :VimwikiColorize'd word showed the literal
`<span style="color:NAME">TEXT</span>` markup. Now the tags are concealed
and TEXT is painted in NAME, so the buffer shows just the coloured word —
matching the HTML export.

New autoload/nuwiki/colors.vim: nuwiki#colors#refresh() scans the buffer
and, per distinct colour, defines a syntax region that conceals the
`<span …>` / `</span>` tags (concealends) and highlights the body with the
span's actual colour (guifg always; ctermfg for named colours). Idempotent
(clears the group before redefining) and tracked in b:nuwiki_color_seen.

Refresh is driven from three places so it's both correct and immediate:
- syntax/vimwiki.vim: initial pass + re-establish after :colorscheme reload
  (resets the per-buffer cache since :syntax clear drops the regions);
- ftplugin TextChanged/InsertLeave autocmd for spans that arrive via paste/
  undo/external edits;
- colorize() itself calls refresh() right after wrapping, so a freshly
  colorized word conceals instantly (TextChanged doesn't fire reliably mid
  command / in headless ex-mode).

The LSP @vimwikiColor token no longer links to Constant, so in Neovim it
doesn't override the real per-span colour on the concealed text. Pure syntax
+ :highlight — works identically in Vim, coc.nvim, and Neovim, no LSP needed.

Tests: colorize.conceal_hides_tags_shows_text (both harnesses) and
colorize.command_conceals_immediately (vim). Sample wiki's colorize section
notes the conceal. nvim 269, vim 259+18, config-parity green.

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

ftplugin/vimwiki.vim

@@ -27,6 +27,16 @@ setlocal concealcursor=c
 
 let b:undo_ftplugin = 'setlocal commentstring< comments< formatoptions< suffixesadd< iskeyword< conceallevel< concealcursor<'
 
+" Conceal `:VimwikiColorize` spans down to their coloured text. New colours
+" appear as the user colorizes, so refresh the syntax groups on edits (and now,
+" for any spans already in the buffer). Both clients — it's pure syntax.
+augroup NuwikiColorConceal
+  autocmd! * <buffer>
+  autocmd TextChanged,TextChangedI,InsertLeave <buffer> call nuwiki#colors#refresh()
+augroup END
+call nuwiki#colors#refresh()
+let b:undo_ftplugin .= ' | execute "autocmd! NuwikiColorConceal * <buffer>"'
+
 if !has('nvim')
   " ===== Plain Vim path =====
   "