Fix custom element check: require keyword arg to disambiguate from variables

Hyphenated names like app-url are variables, not custom elements.
Only treat as custom element when first arg is a Keyword (tag call pattern).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

shared/sx/html.py

@@ -468,8 +468,8 @@ def _render_list(expr: list, env: dict[str, Any]) -> str:
                 return _render_component(val, expr[1:], env)
             # Fall through to evaluation
 
-        # --- Custom element (hyphenated name) → render as tag ------------
-        if "-" in name:
+        # --- Custom element (hyphenated name with keyword attrs) → tag ----
+        if "-" in name and len(expr) > 1 and isinstance(expr[1], Keyword):
             return _render_element(name, expr[1:], env)
 
         # --- SVG/MathML context → unknown names are child elements --------