test: sanity check sQ[0] write+read

tests/unit/test_umma_qk_hd64.cu

@@ -54,6 +54,12 @@ test_umma_qk_hd64(const bf16_t* q, const bf16_t* k,
     }
     __syncthreads();
 
+    // Simple sanity check: thread 0 writes to sQ[0]
+    if (tid == 0) sQ[0] = f32_to_bf16(1.0f);
+    __syncthreads();
+    if (tid == 0) s_out[250] = bf16_to_f32(sQ[0]);  // Should be 1.0
+    __syncthreads();
+
     // Write Q (1, hd) to row 0 of sQ in canonical layout
     for (int d = tid; d < hd; d += N_WARPS * 32) {
         int core_k = d / 8, local_c = d % 8;