feat(editor): BLP→PNG texture export for open format zones

- TextureExporter: converts Blizzard BLP textures to standard PNG
  for fully open redistribution of custom zones
- collectUsedTextures(): finds all texture paths referenced by terrain
- exportTexturesAsPng(): loads BLP via asset manager, writes RGBA PNG
  using stb_image_write to output/MapName/textures/
- Zone export now automatically converts all used textures to PNG
- Client's PNG override system already loads these automatically
  (checks for .png alongside .blp before loading)

Format replacement progress:
- DONE: ADT→WOT/WHM (terrain)
- DONE: WDT→zone.json (map definition)
- DONE: BLP→PNG (textures — auto-exported on zone save)
- TODO: DBC→JSON, M2→open model, WMO→open building

tools/editor/editor_app.cpp

@@ -3,6 +3,7 @@
 #include "zone_manifest.hpp"
 #include "content_pack.hpp"
 #include "wowee_terrain.hpp"
+#include "texture_exporter.hpp"
 #include "core/coordinates.hpp"
 #include "rendering/vk_context.hpp"
 #include "pipeline/adt_loader.hpp"
@@ -731,6 +732,14 @@ void EditorApp::exportZone(const std::string& outputDir) {
         objectPlacer_.saveToFile(objPath);
     }
 
+    // Export used textures as PNG (open format replacement for BLP)
+    auto usedTextures = TextureExporter::collectUsedTextures(terrain_);
+    if (!usedTextures.empty()) {
+        int exported = TextureExporter::exportTexturesAsPng(
+            assetManager_.get(), usedTextures, base + "/textures");
+        LOG_INFO("Exported ", exported, " textures as PNG");
+    }
+
     // Export open terrain format alongside ADT
     std::string openBase = base + "/" + loadedMap_ + "_" +
                            std::to_string(loadedTileX_) + "_" + std::to_string(loadedTileY_);