One syllable per note unless a melisma is explicit

The default word-underlay rule: each note receives exactly one syllable. A single syllable spans more than one note only as a deliberate melisma, marked by an extender line or a slur, never by silent omission.

A vocal line's lyric array has fewer entries than notes with no melisma marker on the gap, so a syllable silently disappears or shifts to align with the wrong note.

Gould 'Lyrics' — normally one syllable is placed under each note; Read.

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