A hairpin must span at least two distinct notes in the correct direction

A crescendo/diminuendo wedge needs a start point and a later end point; a wedge with the end at or before the start is not a hairpin.

`hairpins: [{ from: 5, to: 5, type: "cresc" }]` or `{ from: 7, to: 3, type: "cresc" }` — a same-index or backwards span.

Gould, Behind Bars, ch. Dynamics — hairpins are drawn between two time points that must be distinct and ordered.

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