- mouse-mixin class with on-mouse-down/up/move/scroll handler slots
- handle-mouse-event dispatches to the right handler by event type
- hit-test finds deepest component at (x,y) coordinates
- selection struct + get-selection + copy-to-clipboard
- SGR mouse parsing already existed in input system (mouse-event struct,
parse-sgr-mouse function, CSI dispatch in %read-escape-sequence)
- 3 tests, 100% passing
