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

CMakeLists.txt

@@ -1299,6 +1299,7 @@ add_executable(wowee_editor
     tools/editor/content_pack.cpp
     tools/editor/wowee_terrain.cpp
     tools/editor/editor_project.cpp
+    tools/editor/texture_exporter.cpp
     tools/editor/asset_browser.cpp
     tools/editor/editor_water.cpp
     tools/editor/editor_markers.cpp