Add call-fn dispatch for HO forms: handle both Lambda and native callable

HO forms (map, filter, reduce, etc.) now use call-fn which dispatches
Lambda → call-lambda, native callable → apply, else → clear EvalError.
Previously call-lambda crashed with AttributeError on native functions.

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

shared/sx/tests/test_sx_ref.py

@@ -384,6 +384,29 @@ class TestDefhandler:
         assert adef.params == ["title", "body"]
 
 
+class TestHOWithNativeCallable:
+    def test_map_with_native_fn(self):
+        """map should work with native callables (primitives), not just Lambda."""
+        result = ev('(map str (list 1 2 3))')
+        assert result == ["1", "2", "3"]
+
+    def test_filter_with_native_fn(self):
+        result = ev('(filter number? (list 1 "a" 2 "b" 3))')
+        assert result == [1, 2, 3]
+
+    def test_map_with_env_fn(self):
+        """map should work with Python functions registered in env."""
+        env = {"double": lambda x: x * 2}
+        result = ev('(map double (list 1 2 3))', env)
+        assert result == [2, 4, 6]
+
+    def test_ho_non_callable_fails_fast(self):
+        """Passing a non-callable to map should error clearly."""
+        import pytest
+        with pytest.raises(sx_ref.EvalError, match="Not callable"):
+            ev('(map 42 (list 1 2 3))')
+
+
 class TestMacros:
     def test_defmacro_and_expand(self):
         env = {}