CDSL Bridge: Simple Shaft

Use for a single cylindrical rod, pin, spacer, or stepped shaft, not an assembly.
Invariants: one main axis, one coherent solid, axial segment lengths/diameters, and secondary features referenced to the axis.
Use a circle plus extrude_add_blind for a uniform rod; use a closed side profile plus revolve_add for steps, shoulders, tapers, or grooves.
Add end holes, flats, slots, or keyway-like cuts only after the primary body. Finish last.
Do not invent bearings, gears, couplings, or real thread geometry when not requested.