I dug around and found the latest version of my latest incantation, which is less developed than I thought. It only contains the source of the assembler; there's nothing of the game engine itself. But there's a description of the assembly language and its encoding.

Nothing is uploaded at the moment (I'll start to add things once I've cleaned it up and sorted it out) but I've created a github repository for it https://github.com/alexkus/robotarena if anyone wants to watch it and keep an eye on the (slow) progress.