RectSide
The four visually named sides of a rectangle.
Declaration order is Polygon.Rectangle's counter-clockwise walk — bottom, right, top, left — so on a freshly built rectangle each entry's ordinal is also its edge's index in the rectangle's Polygon.edges. The names themselves are visual, not structural: edges[side] answers by outward direction, so it keeps naming the visually right side after mirrors or negation shuffle the indices; integer indexing stays the structural lane.