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/render/html.rs

@@ -502,6 +502,7 @@ impl HtmlRenderer {
             Keyword::Fixme => ("FIXME", "fixme"),
             Keyword::Fixed => ("FIXED", "fixed"),
             Keyword::Xxx => ("XXX", "xxx"),
+            Keyword::Stopped => ("STOPPED", "stopped"),
         };
         write!(w, "<span class=\"{class}\">{label}</span>")
     }