TermBuf::Events::Resize

The window changed size. The buffer has already been resized to match and everything marked for redraw by the time this arrives.

previous is the size being left. An application that only needs the new geometry can ignore it; one that scales or scrolls to follow the change needs to know which way the window went, and asking the terminal after the fact only ever gives the size it is already at.

This one stays on the terminal side because it carries a ScreenSize, which the input side knows nothing about. It includes Input::Event like the rest and arrives on the same channel.