Here's another program, which also functions as a proof of the Turing-completeness of the virtual PC (well, technically just the same kind of almost-completeness C has, due to the available storage space not being infinite, but I think that's close enough for most purposes).
It's a proof by simulation, in that it implements an interpreter for a programming language which is already known to be Turing-complete, namely brainfuck.
I've tried to follow the implementors recommendations for BF, but the limitations of the underlying system make some of them difficult - as an example, output just goes to the screen in a binary format, instead of being displayed as characters, since there's no native character output and implementing it myself would take too much space (and be a project in its own right).
This program does work in Senbir's default mode, but in that mode it you only have 30 bytes of remaining space to distribute between the BF program and the memory tape it works on, which means you're quite limited in which BF programs you can run with it - and it's not particularly fast in that mode either (I'd guess about the same kind of usable as my Snake game, probably in large part because it uses the same overlay technique (and expects the overlay loader to be used as bootloader)).
However, if you use a mode with a larger disk, all that extra space will be available to the BF program or its tape, without changing the interpreter (beyond changing the BF program). It does not assume anything about the size of the disk (well, except for it being at least 256 words).
Since changing the BF program requires re-preprocessing the source code, for the interpreter to know where the BF program ends and its memory tape begins (it uses labels for this), it's not quite as easily reusable as I might like it to be - but at least it doesn't require manually changing the interpreter code itself.
The interpreter comes with a small example BF program which just reads characters from its input and writes them to its output, in an infinite loop.
Since the input comes from the keyboard and the output goes to the monitor, that allows the player to see what is going on and interact with it, though I'll admit it's not really all that interesting.
In theory you could create a BF implementation of a TC-06 simulator, run it in this interpreter, and then in that simulator run the interpreter again, etc... but I don't think I'd recommend it. :)Since the interpreter source file is 472 lines long, I don't think I'll copy it in here - I'd say it's better to look in the git repository instead (which might even get updated if I find something more to fix).