feat(core): convert Linker.backend from property to classmethod

Make `Linker.backend` callable without instantiation, allowing
downstream libraries (e.g., numba-cuda) to query which linking
backend will be used before constructing a Linker.

The classmethod calls `_decide_nvjitlink_or_driver()` and maps
its return value to "nvJitLink" or "driver". Existing call sites
updated to use parens (`linker.backend()` instead of
`linker.backend`).

Breaking change: attribute-style access `linker.backend` now
returns a bound method, not a string. Only two in-repo call sites
affected; both updated.

Refs #714

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

Author: cpcloud

cuda_core/cuda/core/_linker.pyx

@@ -167,10 +167,22 @@ cdef class Linker:
         else:
             return as_py(self._culink_handle)
 
-    @property
-    def backend(self) -> str:
-        """Return this Linker instance's underlying backend."""
-        return "nvJitLink" if self._use_nvjitlink else "driver"
+    @classmethod
+    def backend(cls) -> str:
+        """Return which linking backend will be used.
+
+        Returns ``"nvJitLink"`` when the nvJitLink library is available
+        and meets the minimum version requirement, otherwise ``"driver"``.
+
+        .. note::
+
+            Prefer letting :class:`Linker` decide. Query ``backend()``
+            only when you need to dispatch based on input format (for
+            example: choose PTX vs. LTOIR before constructing a
+            ``Linker``). The returned string names an implementation
+            detail whose support matrix may shift across CTK releases.
+        """
+        return "driver" if _decide_nvjitlink_or_driver() else "nvJitLink"
 
 
 # =============================================================================

cuda_core/cuda/core/_program.pyx

@@ -649,7 +649,7 @@ cdef inline int Program_init(Program self, object code, str code_type, object op
         self._linker = Linker(
             ObjectCode._init(code.encode(), code_type), options=_translate_program_options(options)
         )
-        self._backend = self._linker.backend
+        self._backend = self._linker.backend()
 
     elif code_type == "nvvm":
         _get_nvvm_module()  # Validate NVVM availability

cuda_core/tests/test_linker.py

@@ -92,7 +92,7 @@ def test_linker_init(compile_ptx_functions, options):
     linker = Linker(*compile_ptx_functions, options=options)
     object_code = linker.link("cubin")
     assert isinstance(object_code, ObjectCode)
-    assert linker.backend == ("driver" if is_culink_backend else "nvJitLink")
+    assert linker.backend() == ("driver" if is_culink_backend else "nvJitLink")
 
 
 def test_linker_init_invalid_arch(compile_ptx_functions):