docs(vehicle): revert the axis fix; the reverse is in the Chaos solver

The axis_forward:X / Blender-Z axle fix changed the reference pose but not the drive
(-2737 cm/s), and axis_forward:X also alters the shared character preset. Revert it.
The Chaos solver drives the wheel about the vehicle's Y (not the bone), and the
drive force is positive yet the car moves backward - the inversion is in the engine
torque / drive-force sign. Next: GetEngineTorque() sign or a SetupVehicle override.

Tools/blender/fbx_export_preset.py

@@ -60,7 +60,6 @@ _METRIC = {
 _CHARACTER = dict(
     _METRIC,
     object_types={"ARMATURE", "MESH"},
-    axis_forward="X",
     add_leaf_bones=False,
     path_mode="COPY",
     embed_textures=False,

Tools/blender/vehicle_build.py

@@ -199,10 +199,8 @@ def build_skeletal(joined, wheels, out_fbx, exp_dir):
     for name, (x, y) in wheel_positions.items():
         bone = edit_bones.new(name)
         bone.head = (x, y, r)
-        # Point the bone along the axle in BLENDER Z: the Unreal FBX import maps (x,y,z)->
-        # (x,-z,-y), so a tail along Z lands on Unreal Y (horizontal axle). Opposite sides
-        # get opposite signs (left down, right up) so the axles point outward.
-        bone.tail = (x, y, r - math.copysign(0.25, y))
+        # Point the bone along the axle: the wheel spins about its own Y.
+        bone.tail = (x, y - math.copysign(0.25, y), r)
         bone.parent = root
     bpy.ops.object.mode_set(mode="OBJECT")