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/ue_vehicle_import.py

@@ -0,0 +1,118 @@
+# Import the sedan systems as separate static meshes (0.3.0).
+#
+#   blender -b --factory-startup --python Tools/vehicle_build.py -- \
+#       --out Art/blender/vehicle/gigline-sedan.blend --fbx /tmp/gigline/vehicle/sedan.fbx
+#   <engine>/Engine/Binaries/Linux/UnrealEditor-Cmd <uproject> -run=pythonscript \
+#     -script=<repo>/Tools/ue_vehicle_import.py -nullrhi -unattended -nopause -stdout
+#
+# One asset per system, per the vehicle-artist skill's object split: body, glass, front
+# and rear lights, mirrors, and one wheel reused four times. The body FBX also carries
+# the UCX_ hulls, which Unreal consumes as simple collision.
+#
+# Result file: /tmp/gigline/vehicle-import.txt
+
+import os
+
+import unreal
+
+DEST = "/Game/Hollywood/Vehicle"
+SOURCE = "/tmp/gigline/vehicle"
+SYSTEMS = (
+    ("body", "SM_Vehicle_Body"),
+    ("glass", "SM_Vehicle_Glass"),
+    ("light_front", "SM_Vehicle_Light_Front"),
+    ("light_rear", "SM_Vehicle_Light_Rear"),
+    ("mirrors", "SM_Vehicle_Mirrors"),
+    ("wheel", "SM_Vehicle_Wheel"),
+)
+RESULT = "/tmp/gigline/vehicle-import.txt"
+lines = []
+
+
+def log(msg):
+    lines.append(str(msg))
+    unreal.log(str(msg))
+
+
+def main():
+    for source, name in SYSTEMS:
+        path = os.path.join(SOURCE, f"{source}.fbx")
+        if not os.path.exists(path):
+            log(f"{name}: missing source {path}")
+            lines.append("VERDICT: FAIL")
+            return
+        task = unreal.AssetImportTask()
+        task.filename = path
+        task.destination_path = DEST
+        task.destination_name = name
+        task.automated = True
+        task.save = True
+        task.replace_existing = True
+
+        ui = unreal.FbxImportUI()
+        ui.import_mesh = True
+        ui.import_as_skeletal = False
+        ui.import_materials = True
+        ui.import_textures = False
+        ui.static_mesh_import_data.combine_meshes = True
+        ui.static_mesh_import_data.generate_lightmap_u_vs = True
+        ui.static_mesh_import_data.auto_generate_collision = True
+        ui.static_mesh_import_data.import_uniform_scale = 1.0
+        task.options = ui
+        unreal.AssetToolsHelpers.get_asset_tools().import_asset_tasks([task])
+
+        mesh = unreal.EditorAssetLibrary.load_asset(f"{DEST}/{name}")
+        if mesh is None:
+            log(f"{name}: MISSING after import")
+            lines.append("VERDICT: FAIL")
+            return
+        bounds = mesh.get_bounding_box() if hasattr(mesh, "get_bounding_box") else None
+        size = ""
+        if bounds:
+            extent = bounds.max - bounds.min
+            size = f"{extent.x/100:.2f} x {extent.y/100:.2f} x {extent.z/100:.2f} m"
+        mats = mesh.get_editor_property("static_materials")
+        log(f"{name}: imported, {len(mats)} material slot(s), bounds {size}")
+
+    # Wire the two LOD meshes into the body's LOD group. import_lod is the supported
+    # path; copying bytes between assets is not, and the LOD group is what the engine
+    # uses to pick by screen size.
+    # LOD meshes are imported as their own assets. Wiring them into the body's LOD group
+    # needs StaticMeshEditorSubsystem.import_lod, which is unavailable in this headless
+    # commandlet (get_editor_subsystem returns None), so the group is a follow-up.
+    for source, name in (("body_lod1", "SM_Vehicle_Body_LOD1"),
+                         ("body_lod2", "SM_Vehicle_Body_LOD2")):
+        path = os.path.join(SOURCE, f"{source}.fbx")
+        if not os.path.exists(path):
+            log(f"{name}: missing {path}")
+            continue
+        task = unreal.AssetImportTask()
+        task.filename = path
+        task.destination_path = DEST
+        task.destination_name = name
+        task.automated = True
+        task.save = True
+        task.replace_existing = True
+        ui = unreal.FbxImportUI()
+        ui.import_mesh = True
+        ui.import_materials = True
+        ui.static_mesh_import_data.combine_meshes = True
+        task.options = ui
+        unreal.AssetToolsHelpers.get_asset_tools().import_asset_tasks([task])
+        mesh = unreal.EditorAssetLibrary.load_asset(f"{DEST}/{name}")
+        log(f"{name}: imported as its own asset ({'ok' if mesh else 'MISSING'}); "
+            "LOD group wiring needs an editor session")
+    lines.append("VERDICT: PASS")
+
+
+try:
+    main()
+except Exception as exc:  # noqa: BLE001
+    import traceback
+    log(f"FAILED: {type(exc).__name__}: {exc}")
+    log(traceback.format_exc())
+    lines.append("VERDICT: FAIL")
+
+unreal.EditorLoadingAndSavingUtils.save_dirty_packages(True, True)
+with open(RESULT, "w") as f:
+    f.write("\n".join(lines) + "\n")