Function register_multibuffer_motions 

pub fn register_multibuffer_motions(
    registry: &mut CommandRegistry,
    mb_registry: MultibufferRegistryHandle,
) -> MultibufferMotionIds

Register the four excerpt-jump motions against registry and return their ids. Handlers capture mb_registry (cheap Arc clone) so they reach the typed view handle via ctx.buffer_id at dispatch time.

Lives in lattice-multibuffer (the crate that owns the excerpt data model). Boot wiring in lattice-host calls this after crate::actions::populate(&mut registry, ...).