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>

syntax/vimwiki.vim

@@ -91,7 +91,7 @@ syntax match nuwikiCodeDelim    /`/ contained conceal
 syntax match nuwikiMathInline   /\$[^$]\+\$/
 " Two keyword groups so the editor can colour them like the HTML export:
 " attention/pending (red) vs resolved/in-progress (green).
-syntax match nuwikiKeywordAttn  /\<\(TODO\|FIXME\|XXX\)\>/
+syntax match nuwikiKeywordAttn  /\<\(TODO\|FIXME\|XXX\|STOPPED\)\>/
 syntax match nuwikiKeywordDone  /\<\(DONE\|FIXED\|STARTED\)\>/
 
 " Wikilink conceal: hide [[ / ]], and for [[target|desc]] hide target|.