UMMA: Rewrite fmha_umma_desc.cuh with correct K-major core-matrix layout + minimal QK GEMM test
- Core-matrix layout: each 8x8 BF16 tile (128B) contiguous in SMEM
- K-major NONE descriptor: LBO=1 (16B), SBO=block_k/8, lbo_mode=0
- MMA K-tiling: tcgen05.mma uses K=16 per call, tile for hd>16
- write_smem_kmajor: converts row-major to core-matrix layout
- write_smem_ktile: extracts single K-tile in core-matrix layout
- test_umma_qk.cu: minimal hd=16, sk=128 test (single MMA call)
- Previous UMMA descriptors were wrong (row-major SMEM, wrong LBO/SBO)