core: clarify usage of classmethods in World class (#1449)

test/general/TestImplemented.py

@@ -22,3 +22,12 @@ class TestImplemented(unittest.TestCase):
                     for region in multiworld.regions:
                         for exit in region.exits:
                             self.assertEqual(exit.parent_region, region)
+
+    def testStageMethods(self):
+        """Tests that worlds don't try to implement certain steps that are only ever called as stage."""
+        for game_name, world_type in AutoWorldRegister.world_types.items():
+            if not world_type.hidden:
+                with self.subTest(game_name):
+                    for method in ("assert_generate",):
+                        self.assertFalse(hasattr(world_type, method),
+                                         f"{method} must be implemented as a @classmethod named stage_{method}.")