Function simplify_mesh 

pub fn simplify_mesh(
    mesh: &Mesh,
    opts: &SimplifyOptions,
) -> (Mesh, SimplifyStats)

Simplify one element mesh. Returns a new mesh carrying the input’s placement/frame metadata (origin, rtc_applied, local_bounds, local_to_world) so world reconstruction (world = origin + position (+ rtc)) and the inverse-placement export path keep working.

A clustered result comes back with an EMPTY normal buffer (the pass changes topology, so the input’s per-face normals no longer apply); consumers rebuild normals when normals.len() != positions.len().