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/handlers.py

@@ -122,7 +122,7 @@ async def execute_handler(
     4. Return rendered string
     """
     from .jinja_bridge import get_component_env, _get_request_context
-    from .async_eval import async_render
+    from .async_eval import async_eval_to_sx
     from .types import NIL
 
     if args is None:
@@ -139,8 +139,9 @@ async def execute_handler(
     # Get request context for I/O primitives
     ctx = _get_request_context()
 
-    # Async eval+render — I/O primitives are awaited inline
-    return await async_render(handler_def.body, env, ctx)
+    # Async eval → sx source — I/O primitives are awaited inline,
+    # but component/tag calls serialize to sx wire format (not HTML).
+    return await async_eval_to_sx(handler_def.body, env, ctx)
 
 
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