ComboBox renders a field showing the selection and a drop-down menu built
entirely with gpui's anchored + deferred elements (no platform primitives).
Menu dismissal is mouse-up-out based so an option click is never swallowed
by a capture-phase close that would redraw the menu away; the field toggles
from the open state recorded at mouse-down. Arrow keys cycle the selection
with wrap-around, escape closes. 48 tests pass.
Mike-Solar
23b1f610a2