A hairpin should arrive at a stated dynamic

A crescendo/diminuendo wedge is conventionally followed immediately by (or ends exactly at) the dynamic level it was building toward; an open-ended hairpin with no destination dynamic is incomplete.

`hairpins: [{ from: 2, to: 9, type: "cresc" }]` where neither note 9 nor note 10 (the next note after the span) carries any `dynamic` value at all — the crescendo has no announced destination loudness.

Gould, Behind Bars, ch. Dynamics — a hairpin's end normally coincides with a new dynamic mark; Ross likewise shows hairpins terminating at an explicit dynamic in virtually every worked example.

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