feat(editor): zone manifest for client loading, export workflow complete

- Zone manifest (zone.json): generated on export with map name, map ID,
  tile list, biome, creature flag, and file paths. This is what the
  wowee client will read to discover and load custom zones.
- Export workflow now produces a complete loadable zone package:
  zone.json + MapName.wdt + MapName_X_Y.adt + creatures.json
- ZoneManifest class with save/load (JSON format)
- Custom map IDs start at 9000+ to avoid conflicting with retail maps
- New Terrain dialog shows helper text for map name format

tools/editor/editor_app.cpp

@@ -1,5 +1,6 @@
 #include "editor_app.hpp"
 #include "adt_writer.hpp"
+#include "zone_manifest.hpp"
 #include "rendering/vk_context.hpp"
 #include "pipeline/adt_loader.hpp"
 #include "pipeline/terrain_mesh.hpp"
@@ -596,6 +597,15 @@ void EditorApp::exportZone(const std::string& outputDir) {
         npcSpawner_.saveToFile(npcPath);
     }
 
+    // Write zone manifest (for client loading)
+    ZoneManifest manifest;
+    manifest.mapName = loadedMap_;
+    manifest.displayName = loadedMap_;
+    manifest.tiles.push_back({loadedTileX_, loadedTileY_});
+    manifest.hasCreatures = (npcSpawner_.spawnCount() > 0);
+    manifest.baseHeight = terrain_.chunks[0].position[2];
+    manifest.save(base + "/zone.json");
+
     lastSavePath_ = outputDir;
     showToast("Zone exported to " + base);
     LOG_INFO("Zone exported to: ", base);