Add VIA2 socket support for external I/O integration

[Russell-S-Harper]

Adds optional -via2-socket support to allow external processes to communicate with the emulator via a UNIX domain socket.

The feature is disabled by default and enabled with -via2 -via2-socket <socket>.

Included:

• Minimal, non-blocking socket integration
• Example client/server implementation
• Documentation in README

This enables experimentation with external tools, automation, and research workflows without affecting core emulation behavior.

[irmen]

I've built and tested it successfully. (Linux).
I kinda like the feature, because it allows for bidirectional communication, whereas the existing Emulator debug registers only allow for writing to the console from the emulator.
Maybe the documentation can be more specific about what VIA register is used to read/write data.

[Russell-S-Harper]

I've built and tested it successfully. (Linux). I kinda like the feature, because it allows for bidirectional communication, whereas the existing Emulator debug registers only allow for writing to the console from the emulator. Maybe the documentation can be more specific about what VIA register is used to read/write data.

I'm glad you like it. In the first draft if -via2 -via2-socket <socket> was specified, then reads or writes to any VIA2 register would read or write to the socket. I revised it so that socket actions are associated only the with first two bytes of the block, VIA ports A and B, with both ports bi-directional. The other registers were left as-is, should the user want to emulate a VIA connection. The README has been revised.

[Russell-S-Harper]

Just a heads up that I'm using the content of this pull request to great effect by integrating an LLM into a game running on the X16 emulator!

Link to writeup and sample videos: https://pvp-ai.russell-harper.com/#v3

[stefan-b-jakobsson]

Testing to approve PR