CUB: ill-formed class-head name in device_transform.cuh breaks GCC (CUDA 13.2)

## Bug type
Compile-time Error

## Affected component
CUB

## Bug description

Commit [1e3b0b4](https://github.com/NVIDIA/cccl/commit/1e3b0b4ce47acdf7fa87674535c4b1a5573aa6f9)
PR: https://github.com/NVIDIA/cccl/pull/6844

("Avoid use of cccl namespace macros in cub") removed `_CCCL_BEGIN_NAMESPACE_CUDA` /
`_CCCL_END_NAMESPACE_CUDA` macros and replaced them with inline explicit qualification.
The resulting code in `cub/device/device_transform.cuh:44` uses `::cuda::` as the
class-head name in a template specialization:

```cpp
struct ::cuda::proclaims_copyable_arguments>
: ::cuda::std::true_type
```

This is **ill-formed C++**. The C++ standard forbids a globally-qualified name (`::cuda::`)
as the class-head name of a struct/class definition or template specialization. The leading
`::` is valid in expression and type-specifier contexts, but **not in a class-head name**.

NVCC on x86 silently accepts this as a non-conforming extension, but **GCC on ARM/SBSA
treats it as a hard error**, breaking any downstream project that includes ``
when building with CUDA 13.2+ on SBSA.

## Error message

```
cub/device/device_transform.cuh:44:134:
error: global qualification of class name is invalid before ':' token
44 | struct ::cuda::proclaims_copyable_arguments<...> : ::cuda::std::true_type
| ^
error: expected '{' before ':' token
```

## Reproducer

Run the shell script below — it creates two CMake projects and a Dockerfile, then
launches `docker build` using the exact image that ships the buggy headers.
The good project compiles cleanly; the bad project fails with the GCC error above.

```bash
#!/bin/bash
set -e

rm -rf /tmp/cccl_reproducer_bad /tmp/cccl_reproducer_good
mkdir -p /tmp/cccl_reproducer_bad /tmp/cccl_reproducer_good

cat << 'REPRODUCER_BAD_EOF' > /tmp/cccl_reproducer_bad/reproducer.cpp
#include

namespace cuda {
template
struct proclaims_copyable_arguments : std::false_type {};
}
namespace detail {
template struct __return_constant {};
}

// ill-formed: leading :: forbidden in class-head name
// mirrors cub/device/device_transform.cuh:44 after commit 1e3b0b4
template
struct ::cuda::proclaims_copyable_arguments>
: std::true_type {};

int main() {}
REPRODUCER_BAD_EOF

cat << 'REPRODUCER_GOOD_EOF' > /tmp/cccl_reproducer_good/reproducer.cpp
#include

namespace cuda {
template
struct proclaims_copyable_arguments : std::false_type {};
}
namespace detail {
template struct __return_constant {};
}

// correct: relative qualification in class-head name
// one-character fix: remove leading :: from class-head name
template
struct cuda::proclaims_copyable_arguments>
: std::true_type {};

int main() {}
REPRODUCER_GOOD_EOF

cat << 'CMAKE_EOF' > /tmp/cccl_reproducer_bad/CMakeLists.txt
cmake_minimum_required(VERSION 3.20)
project(cccl_reproducer_bad CXX)
set(CMAKE_CXX_STANDARD 17)
set(CMAKE_CXX_STANDARD_REQUIRED ON)
message(STATUS "Compiler: ${CMAKE_CXX_COMPILER_ID} ${CMAKE_CXX_COMPILER_VERSION}")
find_package(CUDA REQUIRED)
add_executable(reproducer reproducer.cpp)
CMAKE_EOF

cat << 'CMAKE_GOOD_EOF' > /tmp/cccl_reproducer_good/CMakeLists.txt
cmake_minimum_required(VERSION 3.20)
project(cccl_reproducer_good CXX)
set(CMAKE_CXX_STANDARD 17)
set(CMAKE_CXX_STANDARD_REQUIRED ON)
message(STATUS "Compiler: ${CMAKE_CXX_COMPILER_ID} ${CMAKE_CXX_COMPILER_VERSION}")
find_package(CUDA REQUIRED)
add_executable(reproducer reproducer.cpp)
CMAKE_GOOD_EOF

cat > /tmp/Dockerfile.cccl_reproducer << 'DOCKERFILE_EOF'
FROM nvcr.io/nvidia/cuda-dl-base:26.04-cuda13.2-devel-ubuntu24.04

COPY ./cccl_reproducer_good /cccl_reproducer_good
COPY ./cccl_reproducer_bad /cccl_reproducer_bad

RUN apt-get update -qq && apt-get install -y cmake build-essential

RUN echo -e " ==Device Transform Cuh:44-50== " && echo -e " $(sed -n '40,50p' /usr/local/cuda/include/cccl/cub/device/device_transform.cuh) "

WORKDIR /cccl_reproducer_good
RUN cmake -B /cccl_build_good -S /cccl_reproducer_good && cmake --build /cccl_build_good

WORKDIR /cccl_reproducer_bad
RUN cmake -B /cccl_build_bad -S /cccl_reproducer_bad && cmake --build /cccl_build_bad
DOCKERFILE_EOF

docker build --progress=plain -t cccl_reproducer -f /tmp/Dockerfile.cccl_reproducer /tmp
```

## Suggested fix

```diff
- struct ::cuda::proclaims_copyable_arguments>
+ struct cuda::proclaims_copyable_arguments>
: ::cuda::std::true_type
```

Same fix applies to any other class-head names introduced by the same commit
that use `::cuda::` or `::cuda::std::` as the leading qualifier.

## Environment

| | |
|---|---|
| CCCL version | Introduced in commit 1e3b0b4, present in CUDA 13.2 |
| Affected file | `cub/device/device_transform.cuh:44` |
| Failing compiler | GCC 13+ |
| Reproducer image | `nvcr.io/nvidia/cuda-dl-base:26.04-cuda13.2-devel-ubuntu24.04` |