feat(editor): heightmap export, help overlay, keyboard reference

- Export Heightmap: File > Export Heightmap saves terrain as 16-bit
  RAW grayscale (129x129) for use in external terrain editors or
  as a backup. Configurable max height scale.
- Help overlay (F1 or Help menu): lists all keyboard shortcuts
  organized by category (navigation, editing, object transform, view)
- Round-trip heightmap workflow: import → edit → export

tools/editor/editor_ui.hpp

@@ -39,6 +39,7 @@ private:
     bool showNewDialog_ = false;
     bool showLoadDialog_ = false;
     bool showSaveDialog_ = false;
+    bool showHelp_ = false;
 
     char newMapNameBuf_[256] = "CustomZone";
     int newTileX_ = 32;