fix(packaging): retain source distribution build inputs

Prepared with AI assistance; CPU source-archive regression checks passed.

MANIFEST.in

@@ -0,0 +1,2 @@
+include requirements.txt
+recursive-include csrc *.cpp *.hpp

tests/test_sdist.py

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+"""Check source archive inputs without compiling CUDA or Ascend extensions."""
+
+from __future__ import annotations
+
+import shutil
+import subprocess
+import sys
+import tarfile
+from pathlib import Path
+
+import pytest
+
+# Keep the real setup.py and setuptools commands. Only accelerator discovery
+# and extension construction are substituted, so this test needs no SDK or GPU.
+_SETUP_SCRIPT = """
+import os
+import runpy
+import sys
+import types
+
+from setuptools import Extension
+from setuptools.command.build_ext import build_ext
+
+backend, command = sys.argv[1:]
+for name in (
+    "torch", "torch.utils", "torch.utils.cpp_extension",
+    "torch_npu", "torch_npu.utils", "torch_npu.utils.cpp_extension",
+    "deep_cluster", "deep_cluster.hardware",
+):
+    sys.modules[name] = types.ModuleType(name)
+sys.modules["torch"]._C = types.SimpleNamespace(_GLIBCXX_USE_CXX11_ABI=True)
+ext = sys.modules["torch.utils.cpp_extension"]
+ext.CUDA_HOME = "/unused/cuda"
+ext.CUDAExtension = Extension
+ext.BuildExtension = build_ext
+sys.modules["torch_npu.utils.cpp_extension"].NpuExtension = Extension
+sys.modules["deep_cluster.hardware"].accelerator_kind = lambda: backend
+os.environ["VIRTUAL_ENV"] = "/unused/venv"
+sys.argv = ["setup.py", command]
+runpy.run_path("setup.py", run_name="__main__")
+"""
+
+
+def _run_setup(source: Path, backend: str, command: str) -> None:
+    result = subprocess.run(
+        [sys.executable, "-c", _SETUP_SCRIPT, backend, command],
+        cwd=source,
+        capture_output=True,
+        text=True,
+        check=False,
+    )
+    assert result.returncode == 0, result.stdout + result.stderr
+
+
+@pytest.mark.parametrize("backend", ["nvidia", "ascend"])
+def test_source_archive_contains_build_inputs(tmp_path: Path, backend: str) -> None:
+    root = Path(__file__).resolve().parents[1]
+    source = tmp_path / "source"
+    shutil.copytree(
+        root,
+        source,
+        ignore=shutil.ignore_patterns(".git", "*.egg-info", "__pycache__", "build", "dist", "vendor"),
+    )
+    _run_setup(source, backend, "sdist")
+    archives = list((source / "dist").glob("*.tar.gz"))
+    assert len(archives) == 1
+    with tarfile.open(archives[0]) as archive:
+        members = {name.split("/", 1)[-1] for name in archive.getnames()}
+        required = {"requirements.txt"}
+        required.update(path.relative_to(root).as_posix() for path in (root / "csrc").rglob("*") if path.is_file())
+        assert required <= members, f"Missing source build inputs: {sorted(required - members)}"
+        extracted = tmp_path / "extracted"
+        archive.extractall(extracted, filter="data")
+
+    # A source archive must remain usable without the original checkout, and
+    # contain the inputs for both supported backends regardless of its producer.
+    unpacked = next(extracted.iterdir())
+    for target in ("nvidia", "ascend"):
+        _run_setup(unpacked, target, "egg_info")