Fix _aser_call list serialization causing EvalError on re-parse

Plain Python lists (e.g. from map) were serialized as ((item1) (item2))
which re-parses as a function application, causing "Not callable: _RawHTML"
when the head gets fully evaluated. Keyword list values now wrap as
(<> item1 item2) fragments; positional list children are flattened.

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

shared/sx/async_eval.py

@@ -1249,12 +1249,29 @@ async def _aser_call(
                         extra_class = val.class_name
                     else:
                         parts.append(f":{arg.name}")
-                        parts.append(serialize(val))
+                        # Plain list (e.g. from map) → wrap as fragment to
+                        # avoid ambiguity with function application on re-parse
+                        if isinstance(val, list):
+                            items = [serialize(v) for v in val
+                                     if v is not NIL and v is not None]
+                            parts.append(
+                                "(<> " + " ".join(items) + ")" if items
+                                else "nil"
+                            )
+                        else:
+                            parts.append(serialize(val))
                 i += 2
             else:
                 result = await _aser(arg, env, ctx)
                 if result is not NIL and result is not None:
-                    parts.append(serialize(result))
+                    # Flatten list results (e.g. from map) into individual
+                    # children, matching _aser_fragment behaviour
+                    if isinstance(result, list):
+                        for item in result:
+                            if item is not NIL and item is not None:
+                                parts.append(serialize(item))
+                    else:
+                        parts.append(serialize(result))
                 i += 1
         # If we converted a :style to a class, merge into existing :class or add it
         if extra_class: