add eigen package (#9)

Header-only Eigen 3.4.0 C++ linear algebra library, downloaded from
GitLab release. Exposes INCLUDE_DIR for scons integration.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

eigen/setup.py

@@ -0,0 +1,61 @@
+import os
+import platform
+import subprocess
+
+from setuptools.command.build_py import build_py
+
+try:
+  from wheel.bdist_wheel import bdist_wheel
+except ImportError:
+  bdist_wheel = None
+
+
+class BuildEigen(build_py):
+  """Run build.sh to download Eigen headers before collecting package data."""
+
+  def run(self):
+    pkg_dir = os.path.dirname(os.path.abspath(__file__))
+    marker = os.path.join(pkg_dir, "eigen", "install", "eigen3", "Eigen")
+
+    if not os.path.isdir(marker):
+      build_script = os.path.join(pkg_dir, "build.sh")
+      subprocess.check_call(["bash", build_script], cwd=pkg_dir)
+
+    super().run()
+
+
+cmdclass = {"build_py": BuildEigen}
+
+if bdist_wheel is not None:
+
+  class PlatformWheel(bdist_wheel):
+    """Produce a platform-specific, Python-version-agnostic wheel."""
+
+    def finalize_options(self):
+      super().finalize_options()
+      self.root_is_pure = False
+
+    def get_tag(self):
+      system = platform.system()
+      machine = platform.machine()
+
+      if system == "Linux":
+        plat = f"linux_{machine}"
+      elif system == "Darwin":
+        plat = "macosx_11_0_arm64"
+      else:
+        plat = f"{system.lower()}_{machine}"
+
+      return "py3", "none", plat
+
+  cmdclass["bdist_wheel"] = PlatformWheel
+
+
+def setup():
+  from setuptools import setup as _setup
+
+  _setup(cmdclass=cmdclass)
+
+
+if __name__ == "__main__":
+  setup()