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.hpp

@@ -8,6 +8,7 @@
 #include "object_placer.hpp"
 #include "npc_spawner.hpp"
 #include "npc_presets.hpp"
+#include "zone_manifest.hpp"
 #include "asset_browser.hpp"
 #include "core/window.hpp"
 #include "pipeline/asset_manager.hpp"