Stemlets are dormant because rests never appear inside a beam group here

A stemlet (a short stub stem on a rest inside an otherwise-beamed group, used to keep the beam visually continuous through the rest) is a real named convention, but only applies when beaming-through-rests is itself in use.

N/A under current behavior — a stemlet finding would only be possible if beam-rests were ever enabled without also enabling stemlet rendering.

Stone, Music Notation in the Twentieth Century (names and illustrates the stemlet convention); not used by Gould/Read/Ross, since it belongs to the contemporary departure from the traditional break-at-rest default.

This rule can be decided from pitches, durations and voices — no layout required. A validator can check it before anything is drawn.