feat(editor): texture eyedropper, WOT/WHM preference loading

- Texture eyedropper: pickTextureAt() samples dominant texture at world
  position by reading chunk alpha maps. Alt+Click in paint mode or
  click the Eyedropper button to activate
- loadADT now checks custom_zones/ and output/ for WOT/WHM open format
  files first, falling back to ADT binary only if not found
- Fix unused variable warning in scatterPatches
- Document Alt+Click in help panel

tools/editor/texture_painter.hpp

@@ -42,6 +42,9 @@ public:
     // Erase a texture layer at the given position
     std::vector<int> erase(const glm::vec3& center, float radius, float strength, float falloff);
 
+    // Pick the dominant texture at a world position (eyedropper)
+    std::string pickTextureAt(const glm::vec3& worldPos) const;
+
 private:
     uint32_t ensureTextureInList(const std::string& path);
     int ensureLayerOnChunk(int chunkIdx, uint32_t textureId);