Introduce ClampZone: an axis-aligned rectangular keep-out in plate
coordinates (id + detached Box), validated finite, positive-size and
identified. Plate.ClampZones is a first-class collection alongside
Parts/CutOffs with a ClampZonesChanged event for view refresh. Zones are
never parts: quantity, utilization, cut sequence and CNC output ignore
them (locked by tests); they carry no z-height reasoning.