feat(vehicle): Chaos vehicle spawns and simulates, physics asset generated from script

The physics asset could not be made from Python (PhysicsAssetFactory exposes no mesh
property), so a small editor-only C++ helper calls FPhysicsAssetUtils::CreateFromSkeletalMesh
- the same code path as the editor's Create Physics Asset action. It produces
PA_Vehicle_Sedan with 6 bodies and assigns it to SK_Vehicle_Sedan.

AGigLineVehicle is a Chaos wheeled pawn that configures its four wheels from the modelled
dimensions (radius 33 cm, wheelbase 278, track 156) and is staged in DemoMap. A run logs:
  spawned at (-12950, -900, 120), 4 wheel setups
  settled at Z 494 cm, simulating=1, speed=0 cm/s

That last line is the defect and it is not hidden: the vehicle rests 3.7 m above the road,
so the generated physics bodies do not match the mesh. Recorded in TODO.md with the next
step - inspect the bodies against the mesh, then either fix the creation params or author
two boxes and four wheel spheres explicitly.

Tools/ue_build_demo_map.py

@@ -647,6 +647,17 @@ def build():
           unreal.Rotator(pitch=0.0, yaw=90.0, roll=0.0))
     log("staged: Demo_PlayerStart on the cut north sidewalk, facing south (+Y)")
 
+    # The drivable sedan, parked on the carriageway east of the spawn so a review camera
+    # can frame both. Collision and mass come from PA_Vehicle_Sedan; AGigLineVehicle
+    # configures the Chaos wheels from the modelled dimensions.
+    vehicle_class = unreal.load_class(None, "/Script/GigLine.GigLineVehicle")
+    if vehicle_class:
+        vehicle = spawn(vehicle_class, unreal.Vector(-12950.0, -900.0, 120.0),
+                        "Demo_Vehicle_Sedan")
+        log(f"staged: Demo_Vehicle_Sedan ({'ok' if vehicle else 'FAILED'})")
+    else:
+        log("staged: Demo_Vehicle_Sedan skipped - class not found")
+
     # A lockable LFS .umap checks out read-only, which makes save_map silently
     # return False — chmod u+w first or the staged actors never persist.
     umap = os.path.join(unreal.Paths.project_dir(), MAP.replace("/Game/", "Content/") + ".umap")