Fix clear_parent_cache to also clear root node's cache

clear_parent_cache only cleared self._at_instant_cache inside the
`if self.parent is not None` block, so root nodes (parent=None)
never had their cache cleared. Move the clear() before the
conditional so all nodes in the chain are invalidated.

Confirmed: test_clone_update_invalidates_cloned_cache fails without
this fix and passes with it.

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

Author: MaxGhenis

policyengine_core/parameters/parameter_node.py

@@ -244,6 +244,6 @@ def attach_to_parent(self, parent: "ParameterNode"):
         self.parent = parent
 
     def clear_parent_cache(self):
+        self._at_instant_cache.clear()
         if self.parent is not None:
             self.parent.clear_parent_cache()
-            self._at_instant_cache.clear()