So this is for an all-custom computer, which is rather more impressive on its own (to say nothing of "So I made a C compiler."), but now I'm curious what the minimal implementation of an ethernet card for a "normal" PC would be. I suspect a lot of it would be very similar, up to that you could do checksums on the PC's CPU (probably just baked into the driver). It'd need to be attached - either bare serial or more use…
If a normal PC had an ISA bus (like they did 30 years ago), my network card could have been connected to it with just some minor modifications.
They do apparently have an spi bus inside the pcie standard. Don't know if it would be possible with that or not though.