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_app.cpp

@@ -205,6 +205,7 @@ void EditorApp::processEvents() {
                 auto sc = event.key.keysym.scancode;
                 if (sc == SDL_SCANCODE_F3) setWireframe(!isWireframe());
                 if (sc == SDL_SCANCODE_F5) saveBookmark("");
+                // F1 handled by UI (showHelp_ toggle)
                 // Transform shortcuts (Blender-style)
                 if (objectPlacer_.getSelected()) {
                     if (sc == SDL_SCANCODE_G) startGizmoMode(TransformMode::Move);