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-vim.vim

@@ -515,6 +515,36 @@ call s:record(
       \ 'colorize.ranged_command_no_error',
       \ 'err=' . s:cz_err . ' got ' . string(getline(1)))
 
+" Colour spans are concealed down to their text. Put the span on a non-cursor
+" line so concealcursor (=c) doesn't reveal it, then refresh + inspect.
+call s:set_buf(['plain line', '<span style="color:red">word</span> tail'])
+call cursor(1, 1)
+call nuwiki#colors#refresh()
+let s:cz_l = getline(2)
+let s:cz_open = synconcealed(2, 1)[0]
+let s:cz_word = synconcealed(2, stridx(s:cz_l, 'word') + 1)
+let s:cz_close = synconcealed(2, stridx(s:cz_l, '</span>') + 1)[0]
+call s:record(
+      \ s:cz_open == 1 && s:cz_close == 1 && s:cz_word[0] == 0
+      \   && synIDattr(synID(2, stridx(s:cz_l, 'word') + 1, 1), 'name') ==# 'nuwikiColorSpan_red' ? 1 : 0,
+      \ 'colorize.conceal_hides_tags_shows_text',
+      \ 'open=' . s:cz_open . ' close=' . s:cz_close . ' word=' . s:cz_word[0]
+      \   . ' grp=' . synIDattr(synID(2, stridx(s:cz_l, 'word') + 1, 1), 'name'))
+
+" Running the command conceals the new span immediately (colorize refreshes;
+" no manual step). Move the cursor off the line so concealcursor reveals nothing.
+call s:set_buf(['target word', 'other line'])
+call cursor(1, 1)
+silent VimwikiColorize teal
+call cursor(2, 1)
+let s:cz_l2 = getline(1)
+call s:record(
+      \ synconcealed(1, 1)[0] == 1
+      \   && synIDattr(synID(1, stridx(s:cz_l2, 'target') + 1, 1), 'name') ==# 'nuwikiColorSpan_teal' ? 1 : 0,
+      \ 'colorize.command_conceals_immediately',
+      \ 'concealed=' . synconcealed(1, 1)[0]
+      \   . ' grp=' . synIDattr(synID(1, stridx(s:cz_l2, 'target') + 1, 1), 'name'))
+
 " ===== Follow-link resolves + opens in the current window (regression) =====
 " follow_link_or_create() must resolve the definition itself and `:edit` the
 " target in the CURRENT window — including a page that does not exist yet