refactor: 抽 osmassets 包，要素注册表，材质单一定义源 (P0-P3)

generate_scene.py 从 1271 → 816 行 (-455)

P0: 纯函数搬家
- osmassets/osm.py: parse_osm / Projector / parse_height
- osmassets/geom.py: clip_polygon / point_in_polygon / distance_to_ring / sample_tree_row 等
- blender/tests/test_pure.py: 42 个 unittest (脱离 bpy 运行)

P1: 单一定义源
- osmassets/catalog.py: ROAD_LAYERS + MATERIALS (含 cesium 导出参数)
- 对接 osm2streets_scene_style.json 做图层一致性 warning
- 干掉 road_mats / layer_z / 材质参数三份副本

P2: 要素注册表
- osmassets/{water,grass,scrub}.py: 每个要素一个 assemble() 函数
- build() 中的 if/elif 链收缩为注册表调用
- 计数器集中到 counts 字典

P3: 材质契约化
- catalog.py 扩展 CESIUM_EXPORT 段 (tint/metallic/emission)
- 标记已发现的死条目 Office White Metal Facade (四表各一组)

校验:
- parity.js + scene_digest.py + glb-digest.js 三位一体
- control-1 vs p0/p1/p2a/p2b/p3-counts: 两区域全 PARITY OK
- 42 个纯 Python 测试全部通过

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

blender/osmassets/__init__.py

@@ -0,0 +1,12 @@
+"""Reusable pieces of the OSM → Blender/Cesium asset pipeline.
+
+The package is split by dependency, not by feature:
+
+- `osm`  and `geom` are pure Python.  They import no `bpy` and can be run and
+  tested with a plain interpreter (`python3 -m unittest discover blender/tests`).
+- everything else may touch `bpy` and only runs inside Blender.
+
+Keeping that line sharp is what makes the geometry testable at all; before the
+split it was interleaved with scene construction and could only be exercised by
+rendering a whole area.
+"""