feat: add reimport stage for hand-edited GeoPackages

QGIS 手工修正后的回导流程此前是 README 里的三段 shell：一个硬编码
area-id 和 ogr2ogr 绝对路径的 for 循环、一段内联 node heredoc、再加一次
npm run build。改为一个 reimport 阶段：

  npm run build -- --config config/areas/<area-id>.json \
    --stages reimport,blender,cesium

scripts/reimport-gpkg.js 先把全部图层导出到临时目录并逐个校验，全部通过
才写回 osm2streets_web_out/。ogr2ogr 对不存在的图层退出码非 0 但仍会留下
0 字节文件，原先逐图层 mv 会静默用空图层覆盖好数据。

intermediates 与 reimport 同时指定直接报错——前者用 OSM 重建 GeoPackage，
正好抹掉后者要读回的手工修改。reimport 不含在 all 中。

同时新增 scripts/lib/scene-layers.js 作为 9 个渲染图层的唯一定义源。此前
该表在合并场景、场景样式 JSON、生成的 QGIS 工程、README 手工流程中各有
一份副本，改一处漏其余会导致图层叠放顺序出错并流入 Blender/Cesium。

验证：同一 OSM 输入下，改动前后 intermediates 产物逐字节一致
（scene_style.json、.qgz 符号定义、9 个图层几何与属性）；reimport 对
未修改图层无损回导。

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

scripts/lib/scene-layers.js

@@ -0,0 +1,164 @@
+"use strict";
+
+// Single source of truth for the osm2streets render layers.
+//
+// The same nine layers, in the same order, previously appeared four times:
+// the merged-scene z_index table, the scene style JSON, the generated QGIS
+// project (layer dict + draw_order), and the README's manual rebuild snippet.
+// Adding a layer or changing a z-index meant editing all of them in lockstep,
+// and a missed copy produces a silently mis-stacked scene downstream in
+// Blender/Cesium. Everything now derives from SCENE_LAYERS.
+//
+// zIndex doubles as draw order: lowest paints first (bottom of the stack).
+// outline: null means "no stroke" (QGIS gets a fully transparent outline).
+
+const SCENE_LAYERS = [
+  {
+    id: "road_surface",
+    splitKey: "roadSurface",
+    zIndex: 10,
+    title: "road surface",
+    fill: "#2b2b28",
+    outline: "#1e1e1c",
+    outlineWidth: 0.04,
+  },
+  {
+    id: "intersection_surface",
+    splitKey: "intersectionSurface",
+    zIndex: 20,
+    title: "intersection surface",
+    fill: "#2b2b28",
+    outline: "#1e1e1c",
+    outlineWidth: 0.04,
+  },
+  {
+    id: "sidewalks",
+    splitKey: "sidewalks",
+    zIndex: 30,
+    title: "sidewalks",
+    fill: "#bebeb6",
+    outline: "#9c9c94",
+    outlineWidth: 0.025,
+  },
+  {
+    id: "sidewalk_corners",
+    splitKey: "sidewalkCorners",
+    zIndex: 40,
+    title: "sidewalk corners",
+    fill: "#bebeb6",
+    outline: "#9c9c94",
+    outlineWidth: 0.025,
+  },
+  {
+    id: "lane_separators",
+    splitKey: "laneSeparators",
+    zIndex: 50,
+    title: "lane separators",
+    fill: "#eeeee6",
+    outline: null,
+    outlineWidth: 0,
+  },
+  {
+    id: "center_lines",
+    splitKey: "centerLines",
+    zIndex: 60,
+    title: "center lines",
+    fill: "#f5be2a",
+    outline: null,
+    outlineWidth: 0,
+  },
+  {
+    id: "crosswalks",
+    splitKey: "crosswalks",
+    zIndex: 70,
+    title: "crosswalks",
+    fill: "#fffff6",
+    outline: null,
+    outlineWidth: 0,
+  },
+  {
+    id: "vehicle_stop_lines",
+    splitKey: "vehicleStopLines",
+    zIndex: 80,
+    title: "vehicle stop lines",
+    fill: "#fffff6",
+    outline: null,
+    outlineWidth: 0,
+  },
+  {
+    id: "lane_arrows_webscale",
+    splitKey: "laneArrows",
+    zIndex: 90,
+    title: "lane arrows",
+    fill: "#fffff6",
+    outline: "#2b2b28",
+    outlineAlpha: 200,
+    outlineWidth: 0.015,
+  },
+];
+
+const SCENE_FILE = "osm2streets_scene.geojson";
+const SCENE_STYLE_FILE = "osm2streets_scene_style.json";
+
+function layerFile(layer) {
+  return `${layer.id}.geojson`;
+}
+
+// getCollection(layer) -> FeatureCollection, so callers can source layers from
+// the in-memory split (build) or from disk (reimport) with the same merge.
+function mergeScene(getCollection) {
+  return {
+    type: "FeatureCollection",
+    features: SCENE_LAYERS.flatMap((layer) => {
+      const collection = getCollection(layer) || {};
+      return (collection.features || []).map((feature) => ({
+        ...feature,
+        properties: {
+          ...(feature.properties || {}),
+          render_layer: layer.id,
+          z_index: layer.zIndex,
+        },
+      }));
+    }),
+  };
+}
+
+function sceneStyle() {
+  return {
+    version: 1,
+    geometry: "polygon",
+    sortProperty: "z_index",
+    layerProperty: "render_layer",
+    layers: SCENE_LAYERS.map((layer) => ({
+      id: layer.id,
+      zIndex: layer.zIndex,
+      fill: layer.fill,
+      outline: layer.outline,
+      outlineWidth: layer.outlineWidth,
+    })),
+  };
+}
+
+// QGIS symbol properties want "r,g,b,a" strings rather than hex.
+function qgisRgba(hex, alpha = 255) {
+  if (!hex) return "0,0,0,0";
+  const match = /^#?([0-9a-f]{6})$/i.exec(hex.trim());
+  if (!match) {
+    throw new Error(`Expected #rrggbb color, got: ${hex}`);
+  }
+  const value = parseInt(match[1], 16);
+  const r = (value >> 16) & 0xff;
+  const g = (value >> 8) & 0xff;
+  const b = value & 0xff;
+  return `${r},${g},${b},${alpha}`;
+}
+
+module.exports = {
+  SCENE_LAYERS,
+  SCENE_FILE,
+  SCENE_STYLE_FILE,
+  layerFile,
+  mergeScene,
+  sceneStyle,
+  qgisRgba,
+};