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>

development/tests/test-keymaps.lua

@@ -773,6 +773,22 @@ vim.defer_fn(function()
       error('ranged command did not wrap selection: ' .. got)
     end
   end)
+  -- Colour spans are concealed down to their coloured text. Put the span on a
+  -- non-cursor line so concealcursor (=c) doesn't reveal it, then refresh.
+  tobj_case('colorize.conceal_hides_tags_shows_text', function()
+    set_buf({ 'plain line', '<span style="color:red">word</span> tail' })
+    vim.api.nvim_win_set_cursor(0, { 1, 0 })
+    vim.fn['nuwiki#colors#refresh']()
+    local l = vim.fn.getline(2)
+    local wcol = (l:find('word')) -- 1-based
+    local open = vim.fn.synconcealed(2, 1)[1]
+    local close = vim.fn.synconcealed(2, (l:find('</span>')))[1]
+    local word = vim.fn.synconcealed(2, wcol)[1]
+    local grp = vim.fn.synIDattr(vim.fn.synID(2, wcol, 1), 'name')
+    if not (open == 1 and close == 1 and word == 0 and grp == 'nuwikiColorSpan_red') then
+      error(string.format('open=%s close=%s word=%s grp=%s', open, close, word, grp))
+    end
+  end)
 
   -- ===== Wiki picker (config-driven, no LSP) =====
   -- The picker must build its list from config so it works from any buffer