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>

scripts/glb-digest.js

@@ -0,0 +1,121 @@
+#!/usr/bin/env node
+"use strict";
+
+// Structural digest of a GLB, for the osmassets refactor parity check.
+//
+// Byte-comparing the GLB is too strict: Blender packs images in hash-map order
+// and the buffer padding shifts with it, so two runs of identical code can
+// differ.  This reads the glTF JSON chunk instead and reports the parts that
+// carry meaning downstream in Cesium — node/mesh/material identity and PBR
+// values — plus buffer lengths as a coarse size check.
+//
+//   node scripts/glb-digest.js <file.glb> [--out digest.json]
+
+const fs = require("fs");
+const path = require("path");
+
+function readGlbJson(file) {
+  const buffer = fs.readFileSync(file);
+  if (buffer.length < 12 || buffer.readUInt32LE(0) !== 0x46546c67) {
+    throw new Error(`Not a GLB (bad magic): ${file}`);
+  }
+  const total = buffer.readUInt32LE(8);
+  let offset = 12;
+  while (offset + 8 <= Math.min(total, buffer.length)) {
+    const chunkLength = buffer.readUInt32LE(offset);
+    const chunkType = buffer.readUInt32LE(offset + 4);
+    const start = offset + 8;
+    if (chunkType === 0x4e4f534a) {
+      return JSON.parse(buffer.slice(start, start + chunkLength).toString("utf8"));
+    }
+    offset = start + chunkLength;
+  }
+  throw new Error(`No JSON chunk found in ${file}`);
+}
+
+function round(value) {
+  if (typeof value === "number") return Number(value.toFixed(6));
+  if (Array.isArray(value)) return value.map(round);
+  return value;
+}
+
+function materialDigest(material) {
+  const pbr = material.pbrMetallicRoughness || {};
+  return {
+    name: material.name || null,
+    baseColorFactor: round(pbr.baseColorFactor || null),
+    metallicFactor: round(pbr.metallicFactor ?? null),
+    roughnessFactor: round(pbr.roughnessFactor ?? null),
+    hasBaseColorTexture: Boolean(pbr.baseColorTexture),
+    hasNormalTexture: Boolean(material.normalTexture),
+    emissiveFactor: round(material.emissiveFactor || null),
+    emissiveStrength: round(
+      material.extensions?.KHR_materials_emissive_strength?.emissiveStrength ?? null,
+    ),
+    alphaMode: material.alphaMode || null,
+    doubleSided: material.doubleSided ?? null,
+  };
+}
+
+function digest(file) {
+  const gltf = readGlbJson(file);
+  const meshes = (gltf.meshes || []).map((mesh) => ({
+    name: mesh.name || null,
+    primitives: (mesh.primitives || []).map((primitive) => ({
+      material: primitive.material ?? null,
+      attributes: Object.keys(primitive.attributes || {}).sort(),
+      // Vertex/index counts live on the accessors; they are the real geometry
+      // fingerprint and stay stable regardless of buffer layout.
+      count: gltf.accessors?.[primitive.attributes?.POSITION]?.count ?? null,
+      indices: gltf.accessors?.[primitive.indices]?.count ?? null,
+    })),
+  }));
+  return {
+    file: path.basename(file),
+    fileBytes: fs.statSync(file).size,
+    counts: {
+      nodes: (gltf.nodes || []).length,
+      meshes: meshes.length,
+      materials: (gltf.materials || []).length,
+      images: (gltf.images || []).length,
+      accessors: (gltf.accessors || []).length,
+    },
+    extensionsUsed: (gltf.extensionsUsed || []).slice().sort(),
+    buffers: (gltf.buffers || []).map((buffer) => buffer.byteLength),
+    nodes: (gltf.nodes || [])
+      .map((node) => ({
+        name: node.name || null,
+        mesh: node.mesh ?? null,
+        translation: round(node.translation || null),
+        rotation: round(node.rotation || null),
+        scale: round(node.scale || null),
+        extras: node.extras ?? null,
+      }))
+      .sort((a, b) => String(a.name).localeCompare(String(b.name))),
+    meshes: meshes.sort((a, b) => String(a.name).localeCompare(String(b.name))),
+    materials: (gltf.materials || [])
+      .map(materialDigest)
+      .sort((a, b) => String(a.name).localeCompare(String(b.name))),
+    images: (gltf.images || [])
+      .map((image) => ({ name: image.name || null, mimeType: image.mimeType || null }))
+      .sort((a, b) => String(a.name).localeCompare(String(b.name))),
+  };
+}
+
+const argv = process.argv.slice(2);
+const file = argv.find((arg) => !arg.startsWith("--"));
+if (!file) {
+  console.error("usage: node scripts/glb-digest.js <file.glb> [--out digest.json]");
+  process.exit(1);
+}
+const outIndex = argv.indexOf("--out");
+const result = digest(path.resolve(file));
+const text = `${JSON.stringify(result, null, 2)}\n`;
+if (outIndex >= 0 && argv[outIndex + 1]) {
+  const out = path.resolve(argv[outIndex + 1]);
+  fs.mkdirSync(path.dirname(out), { recursive: true });
+  fs.writeFileSync(out, text);
+  console.log(`GLB digest: ${out}`);
+} else {
+  process.stdout.write(text);
+}