feat(config): P3 config batch — group 4 (valid_html_tags, emoji, color_tag_template)

- valid_html_tags: HTML export now passes the allowlisted inline tags through
  verbatim (b/i/sub/sup/kbd/div/center/strong/em/...) and escapes the rest,
  via a render-time tag-aware escaper (write_escaped_allowing/match_allowed_tag)
  — mirroring upstream's s:safe_html_line regex rather than a parser change.
  `span` is always allowed so colour spans render.
- emoji_enable: render-time `:alias:` -> glyph substitution (curated common
  set), default on; with_emoji builder + substitute_emoji/emoji_for.
- color_tag_template: :VimwikiColorize now wraps via a configurable template
  (g:nuwiki_color_tag_template / setup color_tag_template), splitting on
  __CONTENT__ with __COLORFG__ -> colour; both clients. Colour spans render in
  export via the valid_html_tags span passthrough. Default template reproduces
  the prior `<span style="color:NAME">` output (harness colorize tests green).

Renderer gets with_valid_html_tags + with_emoji; export.rs wires both from
HtmlConfig. Tests: valid_html_tags passthrough + escape, emoji on/off
(html_export.rs). Full rust suite + both harnesses green (Neovim 307,
Vim 301/18/21); clippy clean.

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

autoload/nuwiki/commands.vim

@@ -1085,8 +1085,14 @@ function! nuwiki#commands#colorize(...) abort
   if l:color ==# ''
     return
   endif
-  let l:open_tag = '<span style="color:' . l:color . '">'
-  let l:close_tag = '</span>'
+  " vimwiki `color_tag_template`: split on __CONTENT__ into open/close, with
+  " __COLORFG__ replaced by the colour. Configurable via g:nuwiki_color_tag_template.
+  let l:tpl = get(g:, 'nuwiki_color_tag_template',
+        \ '<span style="color:__COLORFG__">__CONTENT__</span>')
+  let l:tpl = substitute(l:tpl, '__COLORFG__', l:color, 'g')
+  let l:parts = split(l:tpl, '__CONTENT__', 1)
+  let l:open_tag = get(l:parts, 0, '')
+  let l:close_tag = get(l:parts, 1, '')
 
   if l:visual
     let l:sp = getpos("'<")