feat(keywords): add STOPPED keyword

Adds STOPPED alongside TODO/DONE/STARTED/FIXME/FIXED/XXX across the stack:
lexer, AST Keyword enum, HTML renderer (class="stopped"), LSP keyword_str,
and the Vim syntax red group (nuwikiKeywordAttn). Grouped with the
attention/pending keywords (red) for both export and in-editor highlighting.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

crates/nuwiki-core/src/syntax/vimwiki/lexer.rs

@@ -1141,6 +1141,7 @@ const KEYWORDS: &[(&str, Keyword)] = &[
     ("FIXME", Keyword::Fixme),
     ("FIXED", Keyword::Fixed),
     ("XXX", Keyword::Xxx),
+    ("STOPPED", Keyword::Stopped),
 ];
 
 fn match_keyword(rest: &str) -> Option<(usize, Keyword)> {