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/prefill.cu

@@ -1,6 +1,8 @@
 #include "dispatchers.cuh"
 #include "entry_utils.cuh"
 
+using namespace astrai::attention;
+
 torch::Tensor attn_prefill(
     torch::Tensor q,
     torch::Tensor k,