Refactor gfx950 A16W16 GEMM to use the universal kernel

[xytpai]

Summary

• Replace the legacy HGEMM split-K and small-M kernels with a unified gfx950 A16W16 GEMM implementation.
• Support FP16/BF16, NN/NT/TN/TT layouts, bias, FP32 output, slice-K, split-K, and HTI scheduling.
• Add comprehensive correctness tests and restore HGEMM benchmarks, including BF16 8192³ HTI.
• Remove obsolete kernels, tests, and documentation references.

Validation

• gfx950 A16W16 correctness suite
• Concurrent tuner + split-K stress test
• GEMM benchmark suite
• Black and Ruff checks