Complete Cythonization of _program.py

- Reorganize file structure per developer guide (principal class first)
- Add module docstring, __all__, type alias section
- Factor long methods into cdef inline helpers
- Add proper exception specs to cdef functions
- Fix docstrings (use :class: refs, public paths)
- Add type annotations to public methods
- Inline _nvvm_exception_manager (single use)
- Remove Union import, use | syntax
- Add public Program.driver_can_load_nvrtc_ptx_output() API
- Update tests to use new public API

Closes #1082

Author: Andy-Jost

cuda_core/cuda/core/_program.pxd

@@ -7,6 +7,6 @@ cdef class Program:
     cdef:
         object _mnff
         str _backend
-        object _linker  # Linker (not yet Cythonized)
+        object _linker  # Linker
         object _options  # ProgramOptions
         object __weakref__