The public Notation API has no way to author a slur at all

Slurs are fundamental, universal notation; any authoring surface aimed at agents composing real music needs to express phrasing, not just pitch/duration/dynamics/articulation.

Every cited authority (Gould, Read, Ross, Stone) treats slurs as basic vocabulary alongside ties, dynamics, and articulation — their complete absence from an otherwise rich public schema is a scope gap, not a design choice grounded in any convention.

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