Fix defmacro expansion in _aser: check for macros before serializing ~components

The ~component check in _aser immediately serialized all names starting
with ~ as unexpanded component calls. This meant defmacro definitions
like ~root-header-auto were sent to the client unexpanded, causing
"Undefined symbol: root-header-ctx" errors since IO primitives only
exist server-side. Now checks env for Macro instances first.

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

shared/sx/async_eval.py

@@ -1127,8 +1127,12 @@ async def _aser(expr: Any, env: dict[str, Any], ctx: RequestContext) -> Any:
         if name.startswith("html:"):
             return await _aser_call(name[5:], expr[1:], env, ctx)
 
-        # Component call — serialize (don't expand)
+        # Component call — expand macros, serialize regular components
         if name.startswith("~"):
+            val = env.get(name)
+            if isinstance(val, Macro):
+                expanded = _expand_macro(val, expr[1:], env)
+                return await _aser(expanded, env, ctx)
             return await _aser_call(name, expr[1:], env, ctx)
 
         # Serialize-mode special/HO forms (checked BEFORE HTML_TAGS