feat(vehicle): skeletal vehicle asset for Chaos, and the block that follows

Chaos drives a skeletal mesh, so the static system split was not drivable on its own. The
builder now also rigs the vehicle: a root bone plus one bone per wheel, every wheel skinned
to its own bone, exported with the character preset. SK_Vehicle_Sedan imports with
SK_Vehicle_Sedan_Skeleton (508 triangles).

Blocked, and recorded as blocked rather than worked around: Chaos needs a PhysicsAsset, and
unreal.PhysicsAssetFactory in 5.8 exposes no mesh property - setting skeletal_mesh fails
with 'Failed to find property'. Bodies cannot be generated from the mesh in a commandlet,
so the asset has to be authored once in the editor or built in C++ with FPhysicsAssetUtils.

Also fixed the build order: the skeletal join destroys the per-system objects, so the
report and LODs read them before the join - the first attempt died with 'StructRNA of type
Object has been removed'.

TODO.md

@@ -16,7 +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.
+- [A] Set up Chaos Vehicle with wheel and physics configuration. **Blocked on the physics asset, which cannot be generated headless.**
+  - **Skeletal vehicle asset is in** (`SK_Vehicle_Sedan` + `SK_Vehicle_Sedan_Skeleton`): one mesh carrying a root bone and four wheel bones, every wheel skinned to its own bone, exported with the character preset (508 triangles). Chaos drives a skeletal mesh, so the static split alone was not drivable.
+  - **Blocked:** Chaos needs a `PhysicsAsset`, and `unreal.PhysicsAssetFactory` in 5.8 exposes **no mesh property** (`Failed to find property 'skeletal_mesh'`), so the bodies cannot be generated from the skeletal mesh in a commandlet. Options: author it once in the editor and commit it, or build it in C++ with `FPhysicsAssetUtils` in an editor utility.
+  - **Next:** create the physics asset (root box + four wheel capsules at the modelled positions: wheelbase 2.78 m, track 1.62 m, radius 0.33 m), then the `UChaosWheeledVehicleMovementComponent` config reads those same numbers from `Art/blender/vehicle` report.
 - [ ] Implement enter/exit interaction.
 - [ ] Build a closed driving route with boundaries.
 - [ ] Tune basic handling to a drivable state.