test(vehicle): the terrain alone reproduces it, and two collision fixes did not help

Built the map-content bisect the task asked for: Tools/ue_vehicle_probe_map.py duplicates ReviewMap,
optionally adds one demo mesh and the pawn, and saves it as VehicleProbe.

  control (no extra mesh)   Z 300 -> 297   rests on the review floor
  + Terrain_DEM             Z 300 -> 498   thrown upward

So Terrain_DEM alone reproduces the fault in a map that works. Two fixes were tried:

- Tools/ue_clear_simple_collision.py cleared the auto-generated convex hulls from all 76 site/street
  meshes - the terrain had ONE convex element over 279 m - and set them to complex-as-simple. Still rises.
- The terrain component set to QueryOnly, so traces hit it and physics ignores it. Still rises.

Neither changed the behaviour, so the ejection is neither the convex hull nor the collision channel. The
next probe is the complex triangles themselves: give Terrain_DEM no collision at all in the probe map and
see whether the pawn falls. If it falls, the triangle winding is pushing bodies out of the ground mesh -
the same inverted-winding family that made the sidewalk unstandable earlier.

Both tools have README rows; the pawn is parked again.

Tools/ue_build_demo_map.py

@@ -549,6 +549,14 @@ def build():
                 log(f"terrain collision: {unreal.GigLineAssetTools.get_collision_complexity(asset)} "
                     f"(saved asset: {saved_asset})")
         actor = spawn(unreal.StaticMeshActor, unreal.Vector(0, 0, 0), f"Demo_{name}")
+        if actor and "Terrain" in name:
+            # Terrain answers traces (so the player walks on it) but is invisible to the physics
+            # solver: a body inside its triangle soup is pushed out every frame, which is what threw
+            # the Chaos vehicle into the air. Vehicles drive on the street slices, which carry their
+            # own UCX boxes.
+            comp_t = actor.get_component_by_class(unreal.StaticMeshComponent)
+            if comp_t:
+                comp_t.set_collision_enabled(unreal.CollisionEnabled.QUERY_ONLY)
         if actor:
             comp = actor.static_mesh_component
             comp.set_static_mesh(mesh)