A forced down-stem voice keeps its down stem even when its notes rise to the middle line or above it — this is the whole point of the rule, and it is not optional at that boundary

Single-voice notation switches stem direction at the middle line (notes on or above the middle line stem down by default, notes below stem up). Shared-stave writing OVERRIDES this: once a voice is assigned the down-stem role for a passage, its notes keep stemming down even when they climb to or above the middle line — exactly the region where an unforced note would normally flip to up-stems. The forced role wins over the pitch default in every case; there is no pitch height at which the role assignment lapses.

Gould, Behind Bars — the fixed-stem-role convention (see voice-numbering-role-fixation) is explicitly stated as applying regardless of tessitura; Read, Music Notation, describes the same for combined-part writing. This is precisely why the convention exists: an alto voice that rises above a soprano's momentary dip must still read as 'the alto,' which only works if its stems stay down through the crossing.

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