[Afloat16]

The native binding requires topk in [1, 4096], but the Python wrapper allocates output tensors before reaching that check. An invalid large value can therefore trigger an unnecessary allocation or an allocation error.

Reject out-of-range topk before allocating outputs. Values above the vocabulary length remain supported within the existing limit.

Validation:

• 8 CPU wrapper tests pass; 5 fail on the unmodified source.
• Tests also pass under python -O and python -OO.
• Host pytest execution, test lint, Python compilation and git diff --check pass.

This only covers top-k bounds; #15 handles the separate begin/hint/output-dtype checks. Native CUDA/NPU kernels were not executed.