perf: double-buffer KV pipeline and Q direct-to-register in decode

- Double-buffered KV (STAGES=2) for D<=128: next tile cp.async overlaps current tile MMA compute, hiding global load latency
- Q loaded directly from global into mma A-operand registers, removing sQ staging and prologue syncwarp
- Predicated cp.async unifies full and partial tile paths, eliminating scalar fallback branch
- STAGES=1 fallback for D=256 (double-buffer would exceed smem budget)
- Applied to both contiguous and paged decode MMA kernels
- ~1.27x average speedup on L20 (sm_89), zero precision loss

csrc/tests/attn_paged_decode_test.cu

@@ -42,9 +42,10 @@ static void launch_paged_decode(PagedAttentionParams<bf16, float>& p) {
     int G_check = p.q_head / p.kv_head;
     bool use_mma = !p.use_mask && G_check >= 1 && G_check <= 16 && p.page_size >= 32;
     if (use_mma) {
+        constexpr int STAGES = (HEAD_DIM <= 128) ? 2 : 1;
         int tiles_total = (p.kv_len + 32 - 1) / 32;
         p.num_splits = compute_num_splits(p.batch * p.kv_head, tiles_total);
-        paged_attn_decode_split_kv_mma_kernel<HEAD_DIM, 32>
+        paged_attn_decode_split_kv_mma_kernel<HEAD_DIM, 32, STAGES>
             <<<dim3(p.kv_head, p.batch, p.num_splits), 32>>>(p);
     } else
 #endif