fix(IntersectionDetection): handle triangles collapsed to a point

In coplanar segment/triangle detection, best_projection_axis indexed the
triangle vertices with longest_edge_index(), which is NO_LID when all the
triangle vertices are colocated. This out-of-bounds read made
BRepInspector segfault on models with such triangles. The projection axis
now only depends on the segment in that case.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: panquez

src/geode/geometry/intersection_detection.cpp

@@ -106,14 +106,19 @@ namespace
             return max_abs_axis( normal.value() );
         }
         const auto& vertices = triangle.vertices();
+        const auto& segment_vertices = segment.vertices();
+        const geode::Vector3D segment_direction{ segment_vertices[0],
+            segment_vertices[1] };
         const auto longest_edge = triangle.longest_edge_index();
+        if( longest_edge == geode::NO_LID )
+        {
+            return coplanar_projection_axis(
+                segment_vertices[0], segment_direction, { vertices[0] } );
+        }
         const geode::Point3D& origin = vertices[longest_edge];
         const geode::Point3D& next =
             vertices[longest_edge == 2 ? 0 : longest_edge + 1];
         const geode::Vector3D direction{ origin, next };
-        const auto& segment_vertices = segment.vertices();
-        const geode::Vector3D segment_direction{ segment_vertices[0],
-            segment_vertices[1] };
         if( segment_direction.length2() > direction.length2() )
         {
             return coplanar_projection_axis( segment_vertices[0],

tests/geometry/test-intersection-detection.cpp

@@ -224,6 +224,16 @@ void test_segment_degenerated_triangle_intersection_detection()
                 geode::POSITION::parallel, geode::POSITION::parallel ),
         "Wrong result for segment_triangle_intersection_detection with "
         "segment crossing a cap triangle in its plane" );
+
+    const geode::Triangle3D point_triangle{ c, c, c };
+    const auto far_from_point = geode::segment_triangle_intersection_detection(
+        geode::Segment3D{ d, e }, point_triangle );
+    geode::OpenGeodeGeometryException::test(
+        far_from_point
+            == std::make_pair(
+                geode::POSITION::outside, geode::POSITION::outside ),
+        "Wrong result for segment_triangle_intersection_detection with "
+        "triangle collapsed to a point far from the segment" );
 }
 
 void test_segment_line_intersection_detection()