Prove the input path end-to-end. Poll platform events in the frame loop
and, on a mouse_down (UIKit touchesBegan, mapped to an Event in uikit.sx),
toggle the centered quad between orange and green. The toggle marks the
vertex buffer dirty; the next frame re-uploads the active color palette
before drawing. Flip on the press only — a tap also delivers mouse_up
(touchesEnded), so toggling on both would net to no visible change.
goldens/p0_input_before.png (orange quad) and goldens/p0_input_after.png
(green quad) bracket a real tap injected at the device-screen center
(201,437 pt) via `idb ui tap` on the booted iOS 26.0 simulator.
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