fix(vehicle): one chassis body, so the car sits on the road

The vehicle floated 3.7 m above the street because the physics asset had a body per bone
and the four wheel bodies intersected the chassis body, pushing the whole thing up. It is
now a single chassis body on the root bone (452 x 205 x 40 cm) - the same shape Unreal's
vehicle template uses, because Chaos simulates the wheels itself and they must not be
separate rigid bodies.

Settle check, same spawn point:
  before: settled at Z 494 cm
  after:  settled at Z 119 cm

Added -drivetest, which holds full throttle for 3 s and reports speed and distance. It
fails, and the failure names its own cause: BeginPlay logs '0 wheel objects after register'
while the component reports 4 wheel setups, so no wheels exist to receive torque. Recorded
in TODO.md with the next step - check the skeleton bone names against the setups, then give
the front and rear pairs their own wheel subclasses instead of the base class.

TODO.md

@@ -16,11 +16,10 @@ Actionable task list for this project. This file is the single, central place fo
 - [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. - **Imported into Unreal** (`Tools/ue_vehicle_import.py` → `/Game/Hollywood/Vehicle`), split per the vehicle-artist object split: `SM_Vehicle_Body` (3 slots, 4.76 x 1.76 x 1.28 m), `SM_Vehicle_Glass`, `SM_Vehicle_Light_Front`, `SM_Vehicle_Light_Rear`, `SM_Vehicle_Mirrors`, and one `SM_Vehicle_Wheel` (0.22 x 0.66 x 0.66 m) reused four times. The body FBX carries the `UCX_` hulls as simple collision.
   - **LODs cut for real:** decimating the duplicate gives LOD1 at 38 triangles and LOD2 at 14, against 132 for LOD0. The first attempt duplicated the shell unchanged and produced 84-triangle LODs that saved nothing - worth remembering as the failure mode of a "LOD" step nobody measures.
   - **LOD group wiring is open:** `StaticMeshEditorSubsystem.import_lod` is unavailable in a headless commandlet (`get_editor_subsystem` returns None), so the LOD meshes import as their own assets and the group still needs an editor session.
-- [~] Set up Chaos Vehicle with wheel and physics configuration. **The vehicle spawns, simulates and has its four wheel setups; it does not yet sit on the road.**
-  - **Physics asset generated from script.** `unreal.PhysicsAssetFactory` exposes no mesh property, so `UGigLineVehicleBuilder::BuildPhysicsAsset` (C++, editor-only) calls `FPhysicsAssetUtils::CreateFromSkeletalMesh`. Result: `PA_Vehicle_Sedan` with **6 bodies**, assigned to `SK_Vehicle_Sedan`.
-  - **Vehicle class in place.** `AGigLineVehicle` (Chaos wheeled pawn) configures four wheel setups from the modelled dimensions — radius 33 cm, wheelbase 278 cm, track 156 cm — and is staged in DemoMap as `Demo_Vehicle_Sedan`. A run logs `spawned at (-12950, -900, 120), 4 wheel setups`.
-  - **Defect:** it settles at **Z 494 cm** instead of road level (spawned at 120). The physics bodies do not match the mesh, or the car is resting on its own collision boxes — 3.7 m of lift is exactly the kind of thing `bBodyForAll` with generated boxes produces when the boxes are built from the wrong bone extents.
-  - **Next:** inspect `PA_Vehicle_Sedan`'s bodies against the mesh (root box should be ~4.7 x 1.8 x 0.5 m, wheels ~0.66 m), then fix the creation params or replace the auto bodies with two explicit boxes plus four wheel spheres.
+- [~] Set up Chaos Vehicle with wheel and physics configuration. **It sits on the road now; it does not drive yet.**
+  - **Fixed the floating:** the physics asset had a body per bone, and the four wheel bodies intersected the chassis body and pushed the car 3.7 m up. It is now a **single chassis body** on the `root` bone (452 x 205 x 40 cm), which is what Unreal's own vehicle template uses - Chaos simulates the wheels itself, they must not be separate rigid bodies. Settle check: **Z 119 cm** at a spawn of 120 (was 494).
+  - **Drive test fails, with a specific cause:** `-drivetest` holds full throttle for 3 s and reports speed and distance. Result: `speed=0 cm/s, travelled 2.2 m -> FAIL`, and BeginPlay logs **`0 wheel objects after register`** while the component reports 4 wheel setups. The setups exist but no wheels are created from them, so there is nothing to apply torque to.
+  - **Next:** confirm the skeleton's bone names match the setups (`Wheel_FL`, `Wheel_FR`, `Wheel_RL`, `Wheel_RR`) - a name mismatch silently produces zero wheels - and give the front and rear pairs their own `UChaosVehicleWheel` subclasses rather than the base class, which is what the setups point at today.
   - **Also open:** LOD group wiring, enter/exit, driving route, handling.
 - [ ] Implement enter/exit interaction.
 - [ ] Build a closed driving route with boundaries.