test: zero O TMEM before PV GEMM

tests/unit/test_fmha_ts_hd16.cu

@@ -105,6 +105,16 @@ test_fmha_ts(const bf16_t* q, const bf16_t* k, const bf16_t* v,
     }
     __syncthreads();
 
+    // Zero O TMEM region
+    if (wid == 0) {
+        for (int n = 0; n < TMEM_O / 8; n++) {
+            float z0=0,z1=0,z2=0,z3=0,z4=0,z5=0,z6=0,z7=0;
+            asm volatile("tcgen05.st.sync.aligned.32x32b.x8.b32 [%0],{%1,%2,%3,%4,%5,%6,%7,%8};" :: "r"(tb_o+n*8),"f"(z0),"f"(z1),"f"(z2),"f"(z3),"f"(z4),"f"(z5),"f"(z6),"f"(z7));
+        }
+        tmem_fence_store();
+    }
+    __syncthreads();
+
     // STEP 3: PV GEMM via tcgen05.mma TS
     // A = P (TMEM, 128 rows × SK cols), B = V (SMEM K-tiles)
     // C = O (TMEM at tb_o, 128 rows × HD cols)