Reads the terminal and sends decoded keys to a target process.

Two processes are involved. A pump sits blocked in a read on stdin — it can do nothing else, which is exactly why it is on its own. A decoder collects those bytes, runs them through Atui.Key, and forwards {:atui_key, key} messages to the runtime.

The decoder's receive timeout is what makes a bare ESC usable: if no further byte follows within esc_timeout the pending buffer is flushed as :esc rather than waiting forever for an arrow key that is not coming.