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/build-area.js

@@ -15,6 +15,16 @@ const requestedStages = args.stages
   : null;
 const stages = resolveStages(area.stages, requestedStages);
 
+// intermediates deletes and rebuilds the GeoPackage from OSM, which is exactly
+// the manual work reimport exists to recover. Refuse the combination instead of
+// silently letting one undo the other.
+if (stages.intermediates && stages.reimport) {
+  throw new Error(
+    "Stages 'intermediates' and 'reimport' are mutually exclusive: " +
+    "intermediates rebuilds the GeoPackage from OSM and would discard the QGIS edits reimport reads back.",
+  );
+}
+
 console.log(`Area: ${area.id}`);
 console.log(`Config: ${configPath}`);
 console.log(`Output: ${area.outputs.areaDir}`);
@@ -22,6 +32,9 @@ console.log(`Output: ${area.outputs.areaDir}`);
 if (stages.intermediates) {
   buildIntermediates(area);
 }
+if (stages.reimport) {
+  reimportGpkg(area);
+}
 if (stages.blender) {
   buildBlenderScene(area);
 }
@@ -97,6 +110,7 @@ function normalizeAreaConfig(raw) {
       intermediates: raw.stages?.intermediates ?? raw.stages?.qgis ?? true,
       blender: raw.stages?.blender ?? true,
       cesium: raw.stages?.cesium ?? true,
+      reimport: false,
       preview: false,
     },
     qgis: {
@@ -142,6 +156,8 @@ function splitList(value) {
 
 function resolveStages(defaults, requested) {
   if (!requested) return defaults;
+  // 'reimport' is deliberately absent from 'all': it is a recovery step for
+  // hand-edited GeoPackages, never part of a full build.
   const aliases = {
     all: ["intermediates", "blender", "cesium"],
     qgis: ["intermediates"],
@@ -149,6 +165,8 @@ function resolveStages(defaults, requested) {
     geojson: ["intermediates"],
     intermediate: ["intermediates"],
     intermediates: ["intermediates"],
+    reimport: ["reimport"],
+    gpkg: ["reimport"],
     blender: ["blender"],
     scene: ["blender"],
     cesium: ["cesium"],
@@ -157,18 +175,18 @@ function resolveStages(defaults, requested) {
     html: ["preview"],
     cesiumPreview: ["preview"],
   };
-  const out = { intermediates: false, blender: false, cesium: false, preview: false };
+  const out = { intermediates: false, reimport: false, blender: false, cesium: false, preview: false };
   for (const stage of requested) {
     const mapped = aliases[stage];
     if (!mapped) {
-      throw new Error(`Unknown stage '${stage}'. Use intermediates, blender, cesium, preview, or all.`);
+      throw new Error(`Unknown stage '${stage}'. Use intermediates, reimport, blender, cesium, preview, or all.`);
     }
     for (const key of mapped) out[key] = true;
   }
   return out;
 }
 
-function buildIntermediates(area) {
+function writeDerivedConfig(area) {
   fs.mkdirSync(area.outputs.pipelineDir, { recursive: true });
   const derivedConfig = {
     qgisApp: area.qgisApp,
@@ -191,6 +209,11 @@ function buildIntermediates(area) {
   };
   const derivedConfigPath = path.join(area.outputs.pipelineDir, "osm2streets-qgis.config.json");
   fs.writeFileSync(derivedConfigPath, `${JSON.stringify(derivedConfig, null, 2)}\n`);
+  return derivedConfigPath;
+}
+
+function buildIntermediates(area) {
+  const derivedConfigPath = writeDerivedConfig(area);
 
   console.log("Stage: intermediates (osm2streets GeoJSON + QGIS)");
   runCommand(process.execPath, [
@@ -200,6 +223,17 @@ function buildIntermediates(area) {
   ], "intermediates");
 }
 
+function reimportGpkg(area) {
+  const derivedConfigPath = writeDerivedConfig(area);
+
+  console.log("Stage: reimport (GeoPackage -> GeoJSON)");
+  runCommand(process.execPath, [
+    path.join(repoRoot, "scripts", "reimport-gpkg.js"),
+    "--config",
+    derivedConfigPath,
+  ], "reimport");
+}
+
 function buildBlenderScene(area) {
   ensureFile(blenderExecutable(area), "Blender executable");
   ensureFile(path.join(repoRoot, "blender", "generate_scene.py"), "Blender scene generator");