Prepare mdspan for layout_stride_relaxed support - #9877

The NumericTraits specializations for complex<float> and complex<double>
marked them as primitive (is_primitive=true), which caused UB in CUB's
decoupled lookback scan due to torn reads/writes when sizeof(T) >= 8.
The CUB scan path for complex128 was already disabled at the Python
level (cub.pyx:514) since 2020 as a workaround for this very issue.

CUB device reductions (sum, min, max, argmin, argmax) and scans
continue to work correctly for complex types without these traits,
as CuPy provides custom operator specializations and
cuda::std::numeric_limits/is_floating_point_v specializations.

This also unblocks upgrading the CCCL submodule to versions containing
layout_stride_relaxed, where the static_assert in ScanTileState
(introduced by NVIDIA/cccl#7575) would fail for is_primitive types
with sizeof >= 16.

Closes: NVIDIA/cccl#8207

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

Update the CCCL submodule to dee407140b on main, which includes
cuda::layout_stride_relaxed (NVIDIA/cccl#7175). This layout policy
supports negative and zero strides with an explicit offset, enabling
correct mdspan interop for reversed/strided array views.

NOTE: This commit points to a CCCL main commit, not a stable release.
Before merging this PR, the submodule should be updated to the first
stable CCCL release that includes layout_stride_relaxed. Bumping CCCL
is disruptive (invalidates CI cache), so we want to do it only once.

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

The offset field must be packed as ptrdiff_t (8 bytes), not IndexType,
to match the C++ layout_stride_relaxed::mapping's default offset_type.
This matters for IndexType=int32 where sizeof(int)=4 but the C++ struct
expects an 8-byte offset field.

Struct layout (2D example, IndexType=int32):
  [void* ptr (8B)] [int ext0, ext1 (8B)] [int str0, str1 (8B)] [ptrdiff_t offset (8B)]
  Total: 32 bytes = sizeof(void*) + 4*sizeof(int) + sizeof(ptrdiff_t)

Struct layout (2D example, IndexType=int64):
  [void* ptr (8B)] [int64 ext0, ext1 (16B)] [int64 str0, str1 (16B)] [ptrdiff_t offset (8B)]
  Total: 48 bytes = sizeof(void*) + 4*sizeof(int64) + sizeof(ptrdiff_t)

Also fixes:
- _has_layout_stride_relaxed() path to use the installed CCCL symlinks
- Validation test to use layout_stride_relaxed with correct sizeof
- Add skip_no_relaxed_layout to TestMdspanValidation

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

- cimport ptrdiff_t from libc.stddef and use it instead of long long
  for the offset field in _carray.pyx
- Restore original negative-stride tests with layout_stride +
  allow_unsafe=True (legacy path)
- Add separate _relaxed test variants with layout_stride_relaxed
  (safe path, skip_no_relaxed_layout)
- Split TestMdspanValidation into layout_stride (always runs) and
  TestMdspanValidationRelaxed (skip_no_relaxed_layout)
- skip_no_relaxed_layout only applied to tests that exclusively
  target layout_stride_relaxed

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

- Add layout_stride_relaxed to the layout fixture via pytest.param
  with skip_no_relaxed_layout mark, so all existing tests automatically
  cover the relaxed layout
- Merge negative-stride tests: use allow_unsafe conditionally based on
  layout (layout_stride=True, layout_stride_relaxed=False)
- Add both-axes-reversed and sliced-negative-stride tests to
  TestMdspan2D (using the fixture)
- Remove redundant TestMdspanStrideRelaxed class and
  _make_relaxed_kernel_module helper
- Update skip conditions to allow both strided layout variants

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>