Multi-object cross_section for chain-joint verification.

Adds optional object_names: list[str] to blender_cross_section and
blender_cross_section_gallery. Cuts every named object with the same plane
and renders them together — essential for verifying mating-part captivity
(ball-in-socket, peg-in-bore, knuckle interleave) where a single-object cut
can't show whether one part wraps another.

- Cutter cube spans the union bbox of all input objects
- Camera framing uses the union of dup bboxes (post-cut), preserving
  the tight-framing from 0.1.6
- cross_section_gallery passes object_names through to each tile
- Backwards compatible: existing single-object calls work unchanged