[FEA]: Add a small-block segmented-sort kernel using the runtime-width group merge primitive

### Is this a duplicate?

- [x] I confirmed there appear to be no [duplicate issues](https://github.com/NVIDIA/cccl/issues) for this request and that I agree to the [Code of Conduct](CODE_OF_CONDUCT.md)

### Area

CUB

### Is your feature request related to a problem? Please describe.

Fine-grained segmented-sort planning can assign 64, 128, or 256 cooperating threads to a segment. A kernel is needed to consume these small-block work-plan configurations.

### Describe the solution you'd like

Add a small-block segmented-sort kernel that:

- Consumes work-plan configurations using 64, 128, or 256 threads per segment.
- Uses the runtime-width group merge-sort primitive tracked in #10944.
- Supports multiple independent segments per CTA when the configured width permits it.
- Provides correctness coverage.

This issue can be closed once the kernel tested as a consumer of the small-block work-plan tier.

### Describe alternatives you've considered

Separate sorting implementations for each width are unnecessary because they share the runtime-width primitive.

### Additional context

Parent tracking issue: #10946.