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/ast/inline.rs

@@ -14,6 +14,7 @@ pub enum Keyword {
     Fixme,
     Fixed,
     Xxx,
+    Stopped,
 }
 
 #[derive(Debug, Clone, PartialEq, Eq)]