Thanks for the suggestions, @QuasarAlpha!
Robot Overlord is a program to simulate real robots. It can also steer them. The goal is eventually to make a VR training simulator for robots. After starting the app, select World > Add object
Instead of one of the available robots, choose Model.
A placeholder appears at the origin. The model container is selected. clicking in the blank space next to Filename in the context-sensitive menu gives you a file choose dialog. Locate your STL file and click Open. The placeholder will be replaced with your STL model.
In this case the file is quite large, probably to the wrong scale. In a future update some intrepid young coder is going to add context sensitive dialogs to adjust the scale of a model. By double clicking anywhere on the background in 3D you can unselect the model and get back to camera controls. You can also double click on a model to re-select it.