Function framed_transform 

fn framed_transform(zoom: ZoomTransform, crop: CropRect) -> Transform

Build the screen-sprite transform that applies crop then zoom.

The base screen sprite maps a normalized source point (u, v) to NDC (2u − 1, −(2v − 1)) (centre-anchored, scale 2; the − on v is wisp’s +y-up convention — the decoded top-down frame is flipped bottom-up at upload). This composes two affines into the single transform the sprite carries:

1. Crop [x, y, w, h] → fill the sub-rect: scale 2/w, 2/h and recentre so the crop centre lands at NDC (0, 0).
2. Zoom by z about the focal NDC point (2·fx − 1, −(2·fy − 1)), keeping that point fixed: pos += focal · (1 − z).

Composed zoom ∘ crop: scale = z · crop_scale, pos = z · crop_pos + zoom_pos. Pure — no GPU, exhaustively testable.