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/html_renderer.rs

@@ -205,6 +205,7 @@ fn keyword_spans() {
         ("FIXME x\n", "<span class=\"fixme\">FIXME</span>"),
         ("FIXED x\n", "<span class=\"fixed\">FIXED</span>"),
         ("XXX x\n", "<span class=\"xxx\">XXX</span>"),
+        ("STOPPED x\n", "<span class=\"stopped\">STOPPED</span>"),
     ];
     for (src, expected) in cases {
         let out = render(src);