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/tests/lexer.rs

@@ -450,7 +450,7 @@ fn inline_math_captures_content() {
 // ===== Keywords =====
 
 #[test]
-fn all_six_keywords() {
+fn all_keywords() {
     let cases: &[(&str, Keyword)] = &[
         ("TODO", Keyword::Todo),
         ("DONE", Keyword::Done),
@@ -458,6 +458,7 @@ fn all_six_keywords() {
         ("FIXME", Keyword::Fixme),
         ("FIXED", Keyword::Fixed),
         ("XXX", Keyword::Xxx),
+        ("STOPPED", Keyword::Stopped),
     ];
     for (s, expected) in cases {
         let line = format!("{s} stuff\n");