wip(case): refine one-piece enclosure (rib + experiment prints)

Iterate on the single-piece print for #113: update the Blender source and
regenerate the one-shot experiment STLs. Print verification pending —
warping (shrinkage) and visible layer lines are not yet resolved.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: engmung