refactor: namespace csrc kernels and extract common helpers

- attention family -> astrai::attention; fp8 family -> astrai::fp8
- new common/reduce.cuh (warp/group reductions, atomic_max_float)
- new common/cp_async.cuh (predicated cp_async_16, commit/wait group)
- move MAX_SPLITS into attention/common.h; delete warp_utils.cuh
- .cu bindings and pure C tests open family namespaces via using

csrc/kernels/attention/layout_policies.cuh

@@ -26,6 +26,9 @@
 #define DEVICE_FORCEINLINE static __device__ __forceinline__
 #define HOST_DEV_FORCEINLINE static __host__ __device__ __forceinline__
 
+namespace astrai {
+namespace attention {
+
 using bf16 = __nv_bfloat16;
 
 // ============================================================================
@@ -253,3 +256,6 @@ struct PagedKV {
         return kv_addr_from_token(p, c, token, d);
     }
 };
+
+}  // namespace attention
+}  // namespace astrai