[CUCO][FEA]: Make fixed_capacity_map_ref the single source of truth

### 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

CUDA Experimental (cudax)

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

The cudax `fixed_capacity_map` follows the original cuCollections design: the owning implementation provides host bulk operations, while `fixed_capacity_map_ref` provides device operations.

As a result, the owner reconstructs refs for kernel launches, and map state and utilities are split between the owner and ref implementations.

The HyperLogLog migration uses a simpler model: the owner stores its storage and ref, the ref supports both host and device operations, and the owner delegates to it.

### Describe the solution you'd like

Apply the HyperLogLog model to `fixed_capacity_map`:

- Add host bulk operations to `fixed_capacity_map_ref`.
- Make `fixed_capacity_map` own the slot storage and a ref to that storage.
- Forward owner operations and accessors through the stored ref.
- Keep the ref lightweight and device-usable.
- Add tests covering host operations through both the owner and ref.

This would make the ref the single source of truth for map state and shared utilities.

### Describe alternatives you've considered

Keep the current owner/ref split. This works, but continues duplicating state access and operation plumbing.

### Additional context

- [`fixed_capacity_map`](https://github.com/NVIDIA/cccl/blob/main/cudax/include/cuda/experimental/__cuco/fixed_capacity_map.cuh)
- [`hyperloglog`](https://github.com/NVIDIA/cccl/blob/main/cudax/include/cuda/experimental/__cuco/hyperloglog.cuh)
- #4308
- #9867

Start by comparing cudax/include/cuda/experimental/__cuco/fixed_capacity_map.cuh with cudax/include/cuda/experimental/__cuco/hyperloglog.cuh, focusing on storage, references, host operations, and accessors. Trace the existing fixed_capacity_map_ref and owner APIs, then add coverage for host operations through both; done means the ref owns the shared map state and remains lightweight and device-usable.

Written by the indexing model from the issue text.