feat: BF16 sparse SDPA benchmark support and repro script
- bench_sparse_topk.py: add --dtype bf16, the sparse_bf16_kernel_only /
sparse_bf16_e2e modes, and baseline/effective TFLOPS columns. The binding
loader also accepts the ark-xbuild artifact.
- tools/repro_sparse_bf16_sdpa_bench.sh: one-command repro that sources oneAPI
2026.1, pins a device, and runs the BF16 sparse benchmark against the rebuilt
ark-xbuild extension.
