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/osm.py

@@ -0,0 +1,89 @@
+"""OSM XML parsing and the local metric projection.
+
+Pure Python: no `bpy`, so this runs and tests outside Blender.
+"""
+
+import math
+import xml.etree.ElementTree as ET
+
+
+def tags(element):
+    return {t.attrib.get("k", ""): t.attrib.get("v", "")
+            for t in element.findall("tag")}
+
+
+def parse_osm(path):
+    root = ET.parse(path).getroot()
+    bounds_node = root.find("bounds")
+    if bounds_node is None:
+        raise RuntimeError("OSM file does not contain a bounds element")
+    bounds = {"min_lon": float(bounds_node.attrib["minlon"]),
+              "min_lat": float(bounds_node.attrib["minlat"]),
+              "max_lon": float(bounds_node.attrib["maxlon"]),
+              "max_lat": float(bounds_node.attrib["maxlat"])}
+
+    nodes = {}
+    point_features = []
+    for node in root.findall("node"):
+        try:
+            node_id = int(node.attrib["id"])
+            coord = (float(node.attrib["lon"]), float(node.attrib["lat"]))
+            node_tags = tags(node)
+            nodes[node_id] = coord
+            if node_tags:
+                point_features.append({"id": node.attrib.get("id", ""),
+                                       "coord": coord, "tags": node_tags})
+        except (KeyError, ValueError):
+            continue
+
+    ways = []
+    for way in root.findall("way"):
+        if way.attrib.get("action") == "delete":
+            continue
+        refs = []
+        for ref in way.findall("nd"):
+            try:
+                refs.append(int(ref.attrib["ref"]))
+            except (KeyError, ValueError):
+                pass
+        coords = [nodes[r] for r in refs if r in nodes]
+        if len(coords) >= 2:
+            ways.append({"id": way.attrib.get("id", ""),
+                         "coords": coords, "tags": tags(way)})
+    return bounds, ways, point_features
+
+
+def parse_height(feature_tags, default):
+    try:
+        return max(0.5, float(feature_tags.get("height", default)))
+    except ValueError:
+        return default
+
+
+class Projector:
+    """Equirectangular projection about the centre of the OSM bounds.
+
+    Output is metres in a local ENU frame (X east, Y north), which is what both
+    the Blender scene and the Cesium GLB are authored in.
+    """
+
+    def __init__(self, bounds):
+        self.bounds = bounds
+        self.lon0 = (bounds["min_lon"] + bounds["max_lon"]) / 2
+        self.lat0 = (bounds["min_lat"] + bounds["max_lat"]) / 2
+        self.m_per_lat = 111320.0
+        self.m_per_lon = 111320.0 * math.cos(math.radians(self.lat0))
+
+    def xy(self, lon_lat):
+        lon, lat = lon_lat
+        return ((lon - self.lon0) * self.m_per_lon,
+                (lat - self.lat0) * self.m_per_lat)
+
+    def inside(self, lon_lat, pad=0.00035):
+        lon, lat = lon_lat
+        b = self.bounds
+        return (b["min_lon"] - pad <= lon <= b["max_lon"] + pad and
+                b["min_lat"] - pad <= lat <= b["max_lat"] + pad)
+
+    def ring(self, coords):
+        return [self.xy(c) for c in coords]