Linux ↔ Mac support

[mukulchugh]

Support bridging a Mac and a Linux machine.

What this needs

• A Linux client speaking the existing wire protocol (see ARCHITECTURE.md).
• Inject events with uinput (evdev) or an X11/Wayland backend.
• Map macOS keycodes → Linux input event codes.

Why it is a good first issue: isolated new client against a documented protocol; uinput is well-trodden ground and needs no changes to the Mac app to prototype.