Divisi and unison marks

When a stave/voice line temporarily splits into two independently-moving parts, the divergence point is marked 'div.' and the rejoin point 'unis.' — required because a stave with two simultaneous noteheads is otherwise ambiguous between a chord and two independent sub-parts.

Not representable — Gradus's `pitch: string[]` chord notation always means 'these pitches sound as one block for one voice.' MODEL GAP, out of current scope (Gradus teaches composition/theory, not orchestration).

Gould (Behind Bars, divisi/unison marking); Ross (standard abbreviation placement, above the stave, italicized).

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