feat(vehicle): model the sedan with collision hulls

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First 0.3.0 asset. Tools/vehicle_build.py builds a 4.72 x 1.80 x 1.44 m sedan - 2.78 m
wheelbase, 1.62 m track, 0.33 m wheels - as a three-tier silhouette: sill slab, bonnet
and boot shoulders, cabin greenhouse, plus glazing, lamps, bumpers and mirrors. 252
triangles.

Collision is four UCX_-prefixed convex hulls (body, cabin, two bumpers) per the
collision-proxy convention, hidden from render so the preview reads as the car rather
than the colliders.

Two corrections from the preview, both visible in the saved image: the first build was
one tall slab plus a floating cabin and read as a box truck, and the wheels were buried
inside a full-width sill - the sill is now 1.56 m so the wheels stand proud.

Reports every dimension to JSON so the Chaos vehicle config can read the wheelbase, track
and radius instead of restating them.

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