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/vimwiki-gap.md

@@ -140,6 +140,18 @@ fix site.
   `colorize.visual_wraps_selection`, `cmd.Colorize_range_wraps_selection` in
   `test-keymaps.lua` and `colorize.{command_wraps_cword,visual_wraps_selection,
   ranged_command_no_error}` in `test-keymaps-vim.vim`.
+- [x] **Colour spans aren't concealed** — a colorized word shows the literal
+  `<span style="color:…">…</span>` markup instead of just the coloured text.
+  _Fix:_ `autoload/nuwiki/colors.vim` `nuwiki#colors#refresh()` scans the buffer
+  and defines, per colour, a syntax region that conceals the `<span …>`/`</span>`
+  tags (`concealends`) and paints the wrapped text in the span's *actual* colour
+  (`guifg`/`ctermfg`). Wired from the syntax file (initial + on `:colorscheme`
+  reload), an `ftplugin` `TextChanged` autocmd, and directly from `colorize()`
+  for immediate effect. The LSP `@vimwikiColor` token no longer links to
+  `Constant` so it doesn't override the real colour on the concealed text in
+  Neovim. Works in Vim, coc, and Neovim (pure syntax + `:highlight`, no LSP
+  needed). Covered by `colorize.conceal_hides_tags_shows_text` (both harnesses)
+  and `colorize.command_conceals_immediately` (`test-keymaps-vim.vim`).
 - [ ] **`VimwikiTableMoveColumn{Left,Right}` dispatch to different backing names
   per client** — Vim branch → `table_move_left/right`; Neovim branch →
   `table_move_column_left/right`. Harmless today (each name exists in its own