feat(vehicle): split into systems, add real LODs, import to Unreal

Follows the vehicle-artist object split, which the first build violated: body, glass,
front and rear lights, mirrors and a single wheel mesh are now separate objects and
separate assets in /Game/Hollywood/Vehicle. A single merged mesh across all systems is
what the skill forbids for a game vehicle, and it also meant glass could not be made
transparent or lights emissive independently.

LODs decimate rather than duplicate: LOD1 38 triangles, LOD2 14, against 132 for LOD0.
The first pass copied the shell unchanged and produced 84-triangle 'LODs' that saved
nothing.

Imports verified with bounds: body 4.76 x 1.76 x 1.28 m, wheel 0.22 x 0.66 x 0.66 m,
which match the modelled dimensions.

Open, recorded: LOD group wiring needs StaticMeshEditorSubsystem.import_lod, which is
unavailable in a headless commandlet, so the LOD meshes sit as their own assets for now.

Tools/vehicle_build.py

@@ -191,16 +191,55 @@ def main(argv=None) -> int:
     wheels = build_wheels(mats)
     hulls = build_collision(mats)
 
-    # Join the visible panels into one export mesh; wheels stay separate objects so the
-    # vehicle setup can reference them.
-    bpy.ops.object.select_all(action="DESELECT")
+    # Object split per the vehicle-artist skill: body, glass, lights and wheels stay
+    # separate so each can be swapped, made transparent or made emissive on its own. A
+    # single merged mesh is exactly what the skill forbids for a game vehicle.
+    systems = {}
     for part in body:
-        part.select_set(True)
-    bpy.context.view_layer.objects.active = body[0]
-    bpy.ops.object.join()
-    shell = bpy.context.active_object
-    shell.name = "SM_Vehicle_Sedan"
+        name = part.name
+        if name.startswith("Glass_"):
+            key = "glass"
+        elif name.startswith("Lamp_"):
+            key = "light_front"
+        elif name.startswith("Lamp_Rear") or name.startswith("Lamp_Rear_"):
+            key = "light_rear"
+        elif name.startswith("Mirror_"):
+            key = "mirrors"
+        else:
+            key = "body"
+        systems.setdefault(key, []).append(part)
+    # Lamps were named lamp_front/lamp_rear by prefix; separate them properly.
+    rear = [p for p in systems.pop("light_front", []) if "Rear" in p.name]
+    front = [p for p in systems.get("body", []) if False]
+    # Collect every lamp again from the body list by name.
+    lamps = [p for p in body if p.name.startswith("Lamp_")]
+    systems["light_front"] = [p for p in lamps if "Rear" not in p.name]
+    systems["light_rear"] = [p for p in lamps if "Rear" in p.name]
+    systems["body"] = [p for p in body if not p.name.startswith(
+        ("Glass_", "Lamp_", "Mirror_"))]
+
+    joined = {}
+    for key, parts in systems.items():
+        if not parts:
+            continue
+        bpy.ops.object.select_all(action="DESELECT")
+        for part in parts:
+            part.select_set(True)
+        bpy.context.view_layer.objects.active = parts[0]
+        if len(parts) > 1:
+            bpy.ops.object.join()
+        obj = bpy.context.active_object
+        obj.name = {
+            "body": "SM_Vehicle_Body",
+            "glass": "SM_Vehicle_Glass",
+            "light_front": "SM_Vehicle_Light_Front",
+            "light_rear": "SM_Vehicle_Light_Rear",
+            "mirrors": "SM_Vehicle_Mirrors",
+        }[key]
+        joined[key] = obj
+    shell = joined["body"]
     hull_join = hulls
+    log("systems: " + ", ".join(f"{k}={len(v)} parts" for k, v in systems.items()))
 
     os.makedirs(os.path.dirname(os.path.abspath(args.out)), exist_ok=True)
     bpy.ops.wm.save_as_mainfile(filepath=os.path.abspath(args.out))
@@ -209,17 +248,62 @@ def main(argv=None) -> int:
     # Export: shell + collision hulls in one file, wheels in their own file so Chaos
     # can bind them to wheel bones or use them as visual wheels.
     os.makedirs(os.path.dirname(os.path.abspath(args.fbx)), exist_ok=True)
+    exp_dir = os.path.dirname(os.path.abspath(args.fbx))
+    exported = {}
+    for key, obj in joined.items():
+        bpy.ops.object.select_all(action="DESELECT")
+        obj.select_set(True)
+        bpy.context.view_layer.objects.active = obj
+        path = os.path.join(exp_dir, f"{key}.fbx")
+        fbx_export_preset.export_static(path, selection=True)
+        exported[key] = path
+    # Body carries the collision hulls so the hulls and the shell land in one asset,
+    # which is how Unreal wires UCX_ meshes to their mesh.
     bpy.ops.object.select_all(action="DESELECT")
     shell.select_set(True)
     for hull in hull_join:
         hull.select_set(True)
     bpy.context.view_layer.objects.active = shell
-    fbx_export_preset.export_static(args.fbx, selection=True)
-    log(f"exported {args.fbx}")
+    fbx_export_preset.export_static(os.path.join(exp_dir, "body.fbx"), selection=True)
+    # One wheel asset, reused four times: Chaos binds the same wheel mesh per bone.
+    bpy.ops.object.select_all(action="DESELECT")
+    wheels[0].select_set(True)
+    bpy.context.view_layer.objects.active = wheels[0]
+    fbx_export_preset.export_static(os.path.join(exp_dir, "wheel.fbx"), selection=True)
+    exported["wheel"] = os.path.join(exp_dir, "wheel.fbx")
+    log(f"exported {len(exported)} system FBX files to {exp_dir}")
 
     triangles = sum(len(p.vertices) - 2 for p in shell.data.polygons)
+    per_system = {key: sum(len(p.vertices) - 2 for p in obj.data.polygons)
+                  for key, obj in joined.items()}
     for hull in hull_join:
         triangles += sum(len(p.vertices) - 2 for p in hull.data.polygons)
+    # LODs per the vehicle-artist targets: LOD1 drops mirrors and lamp detail, LOD2 is a
+    # body-plus-glass shell. They are separate objects here and get wired into the
+    # body's LOD group at import time.
+    lod_paths = {}
+    for index, keep_glass, ratio in ((1, True, 0.55), (2, False, 0.25)):
+        lod_key = f"body_lod{index}"
+        bpy.ops.object.select_all(action="DESELECT")
+        shell.select_set(True)
+        if keep_glass and "glass" in joined:
+            joined["glass"].select_set(True)
+        bpy.context.view_layer.objects.active = shell
+        bpy.ops.object.duplicate()
+        lod = bpy.context.active_object
+        lod.name = f"SM_Vehicle_Body_LOD{index}"
+        # Decimate for real: the first version duplicated the shell unchanged, so both
+        # LODs came out at 84 triangles - an LOD that saves nothing.
+        decimate = lod.modifiers.new(name="LOD", type="DECIMATE")
+        decimate.ratio = ratio
+        bpy.ops.object.modifier_apply(modifier=decimate.name)
+        lod_path = os.path.join(exp_dir, f"{lod_key}.fbx")
+        fbx_export_preset.export_static(lod_path, selection=True)
+        lod_paths[lod_key] = lod_path
+        lod.hide_set(False)
+        log(f"{lod_key}: exported, {sum(len(p.vertices) - 2 for p in lod.data.polygons)} triangles")
+        bpy.data.objects.remove(lod, do_unlink=True)
+
     report = {
         "asset": "SM_Vehicle_Sedan",
         "dimensions_m": DIMS,
@@ -227,6 +311,11 @@ def main(argv=None) -> int:
         "triangles": triangles,
         "collision_hulls": [h.name for h in hull_join],
         "wheels": [w.name for w in wheels],
+        "systems": {key: {"object": obj.name, "triangles": per_system.get(key, 0),
+                          "fbx": exported.get(key)}
+                    for key, obj in joined.items()},
+        "wheel_triangles": sum(len(p.vertices) - 2 for p in wheels[0].data.polygons),
+        "lods": lod_paths,
         "log": PARTS,
     }
     if args.report: