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>

crates/nuwiki-lsp/src/export.rs

@@ -257,6 +257,10 @@ pub fn render_page_html(
             cfg.html_header_numbering_sym.clone(),
         );
     }
+    if !cfg.valid_html_tags.is_empty() {
+        r = r.with_valid_html_tags(cfg.valid_html_tags.clone());
+    }
+    r = r.with_emoji(cfg.emoji_enable);
     r.render_to_string(doc)
 }