Refactor road assembly module

.trellis/spec/blender/module-structure.md

@@ -25,7 +25,7 @@
      osmassets/materials.py  材质构建
      osmassets/tree.py       树实例化
      osmassets/water.py  grass.py  scrub.py
-     osmassets/building.py  fountain.py       要素装配
+     osmassets/building.py  fountain.py  roads.py       要素装配
      generate_scene.py  export_cesium.py      两个入口脚本
      tools/scene_digest.py                    结构摘要工具
 ```
@@ -54,6 +54,7 @@
 | `water.py` / `grass.py` / `scrub.py` | bpy | 单一 OSM 面要素的装配 |
 | `building.py` | bpy | 单一 OSM 面要素 `building=*` 的装配 |
 | `fountain.py` | bpy | 单一 OSM 点要素 `amenity=fountain` 的装配 |
+| `roads.py` | bpy | osm2streets GeoJSON road layer 装配与 OSM highway 折线回退 |
 
 ### `geom.py` 的两条隐含约定
 
@@ -111,6 +112,18 @@ def assemble(ring, way_id, tag, office_overrides, collection, materials):
 `counts["industrial_count"]`，以及写 `scene["office_override_way_ids"]`。
 模块只负责 prism / roof / windows / bevel / building custom properties。
 
+`roads.py` 是 road layer 装配模块，不拥有图层表或计数：
+
+```python
+def assemble_geojson_layer(path, layer_id, projector, collection, material, z): ...
+def assemble_osm_fallback(ways, projector, collection, material): ...
+```
+
+`generate_scene.py` 仍负责创建 `road_mats`、调用 `catalog.check_layers()` 并打印
+`Layer catalog warning:`、遍历 `catalog.ROAD_LAYERS`、维护 `road_counts`，以及写
+`scene["road_feature_counts"]` / `SCENE_DONE["road_features"]`。模块只负责把已选定的
+GeoJSON layer 或 OSM fallback ways 变成 `Road_<layer_id>` / `OSM_Road_<way_id>` 对象。
+
 ### 加一种新 OSM 要素
 
 目标形态：**新增一个模块 + 注册一行，不改 `build()`**。