refactor: TMA FMHA kernel — 4-warp, proven pattern, full pipeline
Complete rewrite of fmha_6warp_tma.cuh based on lessons learned:
- 128 threads (4 warps) instead of 192 (6 warps) — simpler, proven
- Warp 0: TMA load + softmax, Warp 1: MMA + TMEM alloc
- TMA: mbarrier.arrive.expect_tx (root cause fix), phase parity tracking
- Q loaded directly (T=1 decode), K/V via TMA
- Per-K-sub-tile Q and K loading into (128,16) canonical buffers
- Full softmax + PV GEMM + epilogue pipeline
- Test updated to match new kernel signature
biondizzle
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