refactor: rename gqa_* to attn_*, split-KV for all decode paths

- Rename all csrc/kernels/gqa_*.cuh/cu to attn_*, with _split_q / _split_kv
  strategy suffix and optional _mma compute suffix
- Remove non-split MMA decode kernel, keep only split-KV path
- Convert scalar decode fallback to split-KV (o_part/ml_part + combine)
- Move combine kernel to attn_decode_split_kv.cuh (shared by both paths)
- Rename GQAParams to AttentionParams
- Update all C++ #include, PYBIND11, and Python extension references

astrai/extension/loader.py

@@ -11,7 +11,7 @@ import logging
 
 logger = logging.getLogger(__name__)
 
-KERNEL_NAMES = ["gqa_decode_attn", "gqa_prefill_attn"]
+KERNEL_NAMES = ["attn_decode", "attn_prefill"]
 
 _available: dict[str, bool] = {}
 _modules: dict[str, object] = {}