Species counterpoint exercise — cantus firmus notation and stave assignment

The cantus firmus is notated in whole notes (semibreves) regardless of species, on its own stave (or its own voice line if stacked), and does not receive dynamics, articulations, or ornaments — it is a given melody, not an expressive performance line. The counterpoint stave carries the species' characteristic rhythm. The species number and mode should be identifiable from the exercise, per Fuxian pedagogy as codified in modern editions (Jeppesen, Salzer/Schachter) and echoed by Gould's general 'give every stave its own clear function' principle.

A cantus-firmus voice authored with any duration other than 'w' (whole note), or carrying an `articulation`/`ornament`/`dynamic` field, or a species-counterpoint block where CF and CP are merged into one `notes` array so there is no way to tell which line is the given melody.

Fux (Gradus ad Parnassum) via standard modern species pedagogy (Jeppesen, Salzer & Schachter); Gould for the general engraving principle that a stave's rhythmic identity should be visually unambiguous.

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