add exhibit mockup generator
update dependencies for exhibit-mockup, split generator into separate ns for canopy & plinths
add panelgen ns, refactor canopy & plinth creation, minor updates to core ns
improve curvature in lathe-to-point fn (for plinth canopies)
reduce inset for plinth panels to 2mm, add make-seg-panel6 tree builder for plinths, reduce center extrusion depth from 7.5mm to 5mm