Install igl backend during setup for trimesh booleans

Add an install_igl_backend helper in setup.py that attempts to import igl and, if missing, installs it via pip so that trimesh has a libigl-backed boolean engine available.

Call this helper unconditionally from DownloadAndBuildExt.run, warning but not failing the build if igl installation is not possible, to improve robustness of boolean operations across environments like macOS 3.9.

Author: zasexton

setup.py

@@ -402,6 +402,31 @@ def build_0d(num_cores=None):
     print(f"svZeroDSolver executables have been installed into: {install_tmp_prefix}")
 
 
+def install_igl_backend():
+    """
+    Ensure that the `igl` Python package is available so that trimesh
+    has a robust boolean backend. If `igl` is already importable this
+    is a no-op; otherwise we attempt to install it via pip.
+
+    Any installation failure is converted into a warning so that the
+    overall svv build can still succeed, but boolean operations may
+    lack the libigl-backed engine in that case.
+    """
+    try:
+        import igl  # type: ignore  # noqa: F401
+        return
+    except Exception:
+        pass
+
+    print("Installing 'igl' for trimesh boolean backends...")
+    try:
+        subprocess.check_call([sys.executable, "-m", "pip", "install", "igl"])
+        print("Finished installing 'igl'.")
+    except Exception as e:
+        print(f"Warning: failed to install 'igl' ({e}). "
+              "Trimesh boolean operations may not have a robust backend.")
+
+
 class DownloadAndBuildExt(build_ext):
     def run(self):
         # Make external tool builds opt-in to avoid brittle installs and network fetches
@@ -420,6 +445,10 @@ def run(self):
             except Exception as e:
                 print(f"Warning: svZeroDSolver build failed ({e}). Continuing without building solver.")
 
+        # Always ensure a trimesh/libigl backend is present for boolean operations,
+        # but do not fail the build if installation is not possible.
+        install_igl_backend()
+
         # Always proceed to build Cython extensions
         super().run()