Mechanism sandbox

Free-play curved-arrow checks against authored goldens (instructors / authors). Catalog mechanism problems on the home page use the same engine. Tap a source, then a sink (or press, drag, and release on a sink) to draw an electron-flow arrow (release outside cancels). Multi-step mechanisms (e.g. S_N1) check one step at a time; each new step appears above the previous one, and the products appear at the top when you finish. Click an arrow (or Remove in the list) to delete it; Undo / Clear also work.