I made another progress update to the Boolean Design task. I wrote there:
Now the code to remove triangulation edges after doing the boolean is done. Also, coplanar intersections work. I also did a fairly big refactor to reduce copying of vertex and face data, and to prepare for using floating-point filtering of exact arithmetic predicates (to improve performance). Also did the very first step of performance improvement: using bounding boxes and BVH trees to greatly reduce the number of triangle-triangle intersections tried. The performance is still very far from what I want it to be (currently about 100 times slower than the BMesh boolean on test that intersects two 8k face spheres; and 10 times slower on two 250 face spheres). But I still have a lot of things to do that I know will improve performance.
In terms of functionality, still to do: using original mesh attributes on created new ones (though plumbing is there to make this easy now); hooking up the boolean modifier to this code; doing proper post-operator selection; handling the proper final cutting in the Boolean Knife intersect mode; some handling of non-manifold inputs.