The print/keepsake pipeline's per-bar page-width allocation is a FIXED constant regardless of note density, reproducing the JSX renderer's core defect on the second surface

A system's required width is a function of what its bars actually contain (proportional duration curve + attachments), not a flat width-per-bar constant — the same principle spacing-uniform-system-width-vs-variable-density and bars-per-system-density-driven-not-fixed establish for the JSX surface applies identically here, since the underlying music-notation physics don't change between renderers.

Gould, Behind Bars and Ross, The Art of Music Engraving (density-consistent system layout, cited throughout this chapter) — this rule is the direct print-pipeline analogue of bars-per-system-density-driven-not-fixed, verified by a fresh code read that the chapter's original draft had NOT covered (that draft's summary incorrectly characterized the public API surface as 'narrower risk... mostly about optionsOverride').

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