feat(vehicle): model the sedan with collision hulls

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.

TODO.md

@@ -11,9 +11,9 @@ Actionable task list for this project. This file is the single, central place fo
 
 ## 0.3.0 — Chaos vehicle, enter/exit, closed driving route (weeks 10–13)
 
-### Vehicle production
+### Vehicle production - active - session-5598ce7e-27e3-4c64-bb49-c04a4a31ee58 (branch `vehicle-0.3.0`)
 
-- [ ] Model one vehicle in Blender with collision meshes; export to Unreal.
+- [A] Model one vehicle in Blender with collision meshes; export to Unreal. **Model done** (`Tools/vehicle_build.py` → `Art/blender/vehicle/gigline-sedan.blend`, preview in `Art/blender/vehicle/preview.png`): 4.72 x 1.80 x 1.44 m, 2.78 m wheelbase, 1.62 m track, 0.33 m wheels, 252 triangles, four `UCX_` convex hulls. Dimensions come from real sedan figures and are logged to the report, so the vehicle config can read them rather than restating them. Remaining: import into Unreal (`0.3.0` next step).
 - [ ] Set up Chaos Vehicle with wheel and physics configuration.
 - [ ] Implement enter/exit interaction.
 - [ ] Build a closed driving route with boundaries.