A dynamic under a chord must be centered on the chord's rendered width, not the lowest notehead

A dynamic mark (or any note-attached annotation) is horizontally centered under the full visual width of the notehead(s) it governs — for a chord, centered on the chord's stem/notehead cluster, not left-aligned to whichever pitch the code happens to attach the annotation to.

A rendered dynamic mark on a wide chord (e.g. a 4-note SATB chord spanning a 10th) appears shifted toward one edge of the chord rather than centered under its full notehead cluster, because the underlying renderer attaches the annotation modifier to a single index of the `StaveNote` rather than the chord's bounding box.

Gould, Behind Bars, ch. Dynamics — dynamics are centred below/above the noteheads they belong to; Ross, The Art of Music Engraving, ch. Dynamic Markings, shows centring under the full chord as the default expectation.

This rule is about where marks land on the page, so it can only be judged after engraving. Collision and spacing faults live here.