Either direction: acknowledge receipt and processing of a message.
apps/edge → apps/tutor: device reports its hardware capabilities on connect.
Either direction: keep-alive pulse to detect silent disconnections.
apps/tutor → apps/edge: redraw one or more visual primitives.
apps/tutor → apps/edge: instruct the device to end the session.
apps/tutor → apps/edge: instruct the device to begin a session.
apps/edge → apps/tutor: a completed pen stroke.
String discriminants for every message that can cross the bridge.