[tri_homecart] Visual offset in arm mount stack

[jwnimmer-tri]

See #33 (review) for details, at "FYI the model is flawed":

We stack a number of the mounting plates. However, we have a gap at the bottom of the stack, and an extra plate at the top that is actually intersecting the robot arm's base. The proximity geometry for this box (represented as a single box) is in the correct place.Looking at master, it's clear that's a defect that predates this PR.

The model for the mount stack does not lie on the Wz = 0 plane, it floats above it. Without a keypad, I'm somewhat hampered in my ability to use blender, but it appears to be floating about 0.15 cm above the plane. If the geometry were to be moved (either in the .bin file and moved, or by adding a translation to the node in the .gltf file), I believe the stack would be corrected.