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-lsp/src/lib.rs

@@ -1311,6 +1311,7 @@ fn keyword_str(k: nuwiki_core::ast::Keyword) -> &'static str {
         nuwiki_core::ast::Keyword::Fixme => "FIXME",
         nuwiki_core::ast::Keyword::Fixed => "FIXED",
         nuwiki_core::ast::Keyword::Xxx => "XXX",
+        nuwiki_core::ast::Keyword::Stopped => "STOPPED",
     }
 }