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>

syntax/vimwiki.vim

@@ -54,9 +54,12 @@ if has('nvim')
 
   " Misc.
   highlight default link @vimwikiKeyword              Todo
-  highlight default link @vimwikiColor                Constant
   highlight default link @vimwikiComment              Comment
   highlight default link @vimwikiDefinitionTerm       Statement
+  " NOTE: no default for @vimwikiColor. Colour spans are coloured client-side
+  " in their *actual* colour by nuwiki#colors#refresh() (conceal + per-colour
+  " highlight); linking the LSP token to a fixed group here would override that
+  " real colour on the concealed text in Neovim.
 endif
 
 " ===== Static fallback (no LSP required) =====
@@ -205,4 +208,10 @@ endfunction
 
 call s:nuwiki_setup_nested()
 
+" Conceal `:VimwikiColorize` spans (`<span style="color:…">…</span>`) down to
+" their coloured text. Reset the per-buffer cache first so a syntax reload
+" (e.g. on :colorscheme) re-establishes the regions cleared by `:syntax clear`.
+let b:nuwiki_color_seen = {}
+call nuwiki#colors#refresh()
+
 let b:current_syntax = 'vimwiki'