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/texture_exporter.hpp

@@ -0,0 +1,26 @@
+#pragma once
+
+#include "pipeline/adt_loader.hpp"
+#include <string>
+#include <vector>
+#include <unordered_set>
+
+namespace wowee {
+namespace pipeline { class AssetManager; }
+
+namespace editor {
+
+class TextureExporter {
+public:
+    // Collect all texture paths referenced by the terrain
+    static std::vector<std::string> collectUsedTextures(const pipeline::ADTTerrain& terrain);
+
+    // Export all used textures as PNG to an output directory
+    // Returns count of successfully exported textures
+    static int exportTexturesAsPng(pipeline::AssetManager* am,
+                                    const std::vector<std::string>& texturePaths,
+                                    const std::string& outputDir);
+};
+
+} // namespace editor
+} // namespace wowee