Potential fix for pull request finding

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>
Signed-off-by: Nathaniel Starkman <nstarman@users.noreply.github.com>

Author: nstarman

src/xmmutablemap/_core.py

@@ -99,6 +99,10 @@ def __eq__(self, other: object) -> bool:
             return self._data == dict(other.items())
         return NotImplemented
 
+    def __hash__(self) -> int:
+        """Return an order-insensitive hash based on the mapping contents."""
+        return hash(frozenset(self._data.items()))
+
     # ===========================================
     # Mapping Protocol