Interface RectPose

Axis-aligned rectangle pose with optional rotation. The canonical pose shape used by composeRectPose and the unionBounds helper. Rotation is in radians, pivoted on the unrotated AABB center; absent === 0. Kit-side code that consumes rotation already reads pose.rotation ?? 0 (SceneCanvas.defaultDrawOne, rotate/handle.ts, pathInWorld.ts), so the slot exists on every default scene whether or not the consumer populates it.