A 2D affine transform stored as a 2×3 matrix [a, b, c, d, e, f], mapping a
point (x, y) to (a*x + c*y + e, b*x + d*y + f). This is the same [f32; 6]
layout consumed by RenderNode::transform_with, so to_array() plugs in
directly. Compose with Transform::then instead of multiplying matrices by
hand.