feat: add optional CUDA kernel system (csrc/) + fused GQA decode attention

Structure:
  csrc/               -- .cu sources + build.py registry
  astrai/extension/   -- compiled .so + __init__.py (import dispatcher)
  setup.py            -- CUDAExtension from csrc/build.py REGISTRY

Control: CSRC_KERNELS=true|false env var at install time.
Fallback: astrai.extension.available dict for runtime detection.

setup.py

@@ -0,0 +1,58 @@
+import os
+import sys
+from pathlib import Path
+
+from setuptools import setup
+from setuptools.command.build_ext import build_ext as _build_ext
+
+sys.path.insert(0, str(Path(__file__).parent))
+os.makedirs("astrai/extension", exist_ok=True)
+
+
+def _should_build():
+    force = os.environ.get("CSRC_KERNELS", "").strip().lower()
+    if force == "true":
+        return True
+    if force == "false":
+        return False
+    try:
+        import shutil
+
+        import torch
+
+        return shutil.which("nvcc") is not None and torch.cuda.is_available()
+    except Exception:
+        return False
+
+
+ext_modules = []
+cmdclass = {}
+
+if _should_build():
+    import torch
+    from torch.utils.cpp_extension import BuildExtension, CUDAExtension
+
+    from csrc.build import REGISTRY
+
+    _torch_lib = torch.utils.cpp_extension.library_paths()[0]
+
+    for name, info in REGISTRY.items():
+        ext_modules.append(
+            CUDAExtension(
+                f"astrai.extension.{name}",
+                info["sources"],
+                extra_compile_args={"cxx": ["-O3"], "nvcc": info["nvcc_flags"]},
+                extra_link_args=[f"-Wl,-rpath,{_torch_lib}"],
+            )
+        )
+    cmdclass["build_ext"] = BuildExtension
+
+if not cmdclass:
+
+    class _NullBuildExt(_build_ext):
+        def build_extensions(self):
+            pass
+
+    cmdclass["build_ext"] = _NullBuildExt
+
+setup(ext_modules=ext_modules, cmdclass=cmdclass)