6-warp specialized FMHA kernel — ALL HD=16/64/128/256 PASS cos 0.999997+

Warp layout (192 threads):
- Warps 0-3: Softmax + correction + epilogue
- Warp 4: MMA (QK + PV GEMM)
- Warp 5: Data staging (Q/K/V loads, direct GMEM for now)
CTA-wide __syncthreads() sync between phases.

Fix: removed spurious inv_sum normalization in epilogue
(MMA output is already correctly scaled with softmax'd P).

Files: fmha_6warp.cuh + test_fmha_6warp*.cu

dsv4/kernels/attention/fmha_6warp.cuh

@@ -211,7 +211,6 @@ fmha_6warp_kernel(
     // Epilogue: TMEM → regs → normalize → BF16 → GMEM (warp 0)
     // ================================================================
     if (wid == 0) {
-        float inv_sum = 1.0f / *sRowSum;
         float o_vals[HD];
         for (int n = 0; n < HD / 8; n++) {
             float tmp[8];
@@ -220,7 +219,7 @@ fmha_6warp_kernel(
                   "=f"(tmp[4]),"=f"(tmp[5]),"=f"(tmp[6]),"=f"(tmp[7])
                 : "r"(tb + n*8));
             asm volatile("tcgen05.wait::ld.sync.aligned;");
-            if (lane == 0) for (int c=0;c<8;c++) o_vals[n*8+c] = tmp[c] * inv_sum;
+            if (lane == 0) for (int c=0;c<8;c++) o_vals[n*8+c] = tmp[c];
         }
         if (lane == 0) for (int d=0;d<HD;d++) o[d] = f32_to_bf16(o_vals[d]);
     }