I'm working on improving the hinter and solver, but in the meantime I added some unit tests for the four line rectangle and arcs and fixed some small issues that I discovered while writing these. I've also started working on the spline tests, but they're a bit more complicated and I've postponed them a bit. The changes are here.
I've also fixed a crash that was caused by the sketch editing not being closed before opening a new file. The changes for this are here.