Cross-staff writing — stem direction and beam bridging

When a note or beam group crosses from one stave to another, the stem points TOWARD the destination stave, and the beam is drawn continuously bridging the gap rather than breaking into two separate beam groups.

Not currently authorable — neither the JSX props nor the public API has any concept of a note migrating staves mid-phrase. MODEL GAP: the feature doesn't exist.

Gould (Behind Bars, cross-staff notation chapter, explicit stem-direction-toward-destination rule); Ross covers the mechanical beam-bridging convention.

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