perf: precompute ragged Q tile scheduling

csrc/kernels/attn_common.h

@@ -13,7 +13,7 @@ enum TensorLayout : int {
 //   - Contiguous K/V: dense [batch, kv_head, kv_len, head_dim] tensors (k/v).
 //   - Paged (SGLang-style): flat pool [size, kv_head, head_dim] + req_to_token.
 // Each kernel selects the addressing via a KVSource policy (see
-// attn_kv_source.cuh); a given call only touches the fields of one mode, so
+// attn_layout_policies.cuh); a given call only touches the fields of one mode, so
 // this is a POD shared by both paths rather than two parallel structs that
 // drift out of sync.
 template<typename T, typename AT = float>
@@ -59,6 +59,9 @@ struct AttentionParams {
     const int* __restrict__ req_pool_indices;     // [batch]
     const int* __restrict__ kv_indptr;             // [batch + 1]
     const int* __restrict__ qo_indptr;             // [batch + 1] or nullptr for decode
+    const int* __restrict__ q_tile_to_batch;        // [num_q_tiles], prefill only
+    const int* __restrict__ q_tile_to_index;        // [num_q_tiles], prefill only
+    int num_q_tiles;
     int max_context_len; // req_to_token stride (dim 1)
 
     // Decode split-KV workspace