Fix defhandler to produce sx wire format instead of HTML

execute_handler was using async_render() which renders all the way to
HTML. Fragment providers need to return sx source (s-expression strings)
that consuming apps parse and render client-side.

Added async_eval_to_sx() — a new execution mode that evaluates I/O
primitives and control flow but serializes component/tag calls as sx
source instead of rendering them to HTML. This mirrors how the old
Python handlers used sx_call() to build sx strings.

Also fixed: _ASER_FORMS checked after HTML_TAGS, causing "map" (which
is both an HTML tag and an sx special form) to be serialized as a tag
instead of evaluated. Moved _ASER_FORMS check before HTML_TAGS.

Also fixed: empty? primitive now handles non-len()-able types gracefully.

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

shared/sx/primitives.py

@@ -189,7 +189,10 @@ def prim_is_dict(x: Any) -> bool:
 def prim_is_empty(coll: Any) -> bool:
     if coll is None or coll is NIL:
         return True
-    return len(coll) == 0
+    try:
+        return len(coll) == 0
+    except TypeError:
+        return False
 
 @register_primitive("contains?")
 def prim_contains(coll: Any, key: Any) -> bool: