test(core): assert backend is classmethod not property

Explicit guard that the intentional property-to-classmethod break
is preserved. Documents the breaking change in the test itself.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: cpcloud

cuda_core/tests/test_linker_backend.py

@@ -47,3 +47,13 @@ def test_backend_idempotent(self, monkeypatch):
         monkeypatch.setattr(_linker, "_use_nvjitlink_backend", True)
         results = [Linker.backend() for _ in range(3)]
         assert results == ["nvJitLink", "nvJitLink", "nvJitLink"]
+
+    def test_backend_is_not_property(self):
+        """backend is a classmethod, not a property.
+
+        This is an intentional breaking change from the prior property API.
+        Attribute-style access (``linker.backend``) now returns a bound method,
+        not a string. All call sites must use parens: ``Linker.backend()``.
+        """
+        attr = inspect.getattr_static(Linker, "backend")
+        assert not isinstance(attr, property)