Cartridge stores return value in R0, which is used for message acknowledgement about ycpu

I've just realized that keyboard also sends return value as response in some handler. It's at least consistent on MSG_ERROR. Perhaps I could do the same with cartridge, but it feels weird returning 1 on error, which is MSG_ACK and kind of indicate success.

[ZaneDubya]

@Naryosha Would you like to propose a change to the hardware spec, or are you comfortable leaving as is?

Since mentioning that R0 could be something else rather than MSG_* would simply fix the issue, for now I'm comfortable as it is. I'll mention it in my own docs though because it's still an unofficial decision.

I see there are two ways to properly resolve this:

1. change YCPU specs (mainly section about HWQ $02) to match hardware specs: as written above. Hardware may write to R0, R1, and R2 with anything as defined in their specs.
2. change hardware specs to match YCPU specs: Hardware may only return MSG_* in R0. It also may not modify R1 and R2 if MSG_ERROR is sent via R0.

If I were to resolve it, it'd take some time. 😅