An underfull final system should not be force-justified to the same width as full systems

When a piece ends partway through what would otherwise be a full system's worth of bars, standard practice leaves that final system at its NATURAL (density-consistent) width rather than stretching it to match the page/column width.

Same root cause as spacing-uniform-system-width-vs-variable-density: `staveW` computed once from the block's maximum, reused for every system row. The LAST system is the one most likely to be underfull (total bar count rarely divides evenly by `barsPerSystem`), and it gets identical treatment to every full system.

Gould, Behind Bars (explicit: a short final system is not force-justified); Ross, The Art of Music Engraving broadly agrees. NOTE — some house styles (hymnal/chorale typesetting with strict right-margin alignment across every line) DO force-justify every system including the last, so authorities are not unanimous. RECOMMENDATION for Gradus: follow Gould's convention — Gradus's content is pedagogical excerpts, not fixed-page hymnal layout, and a stretched final system reads as a rendering bug to a parent/student, which is the wrong signal in a design system that prizes precision.

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