Function plan 

fn plan(
    meshes: &[MeshData],
    baked_basis: Option<&Matrix4<f64>>,
    prior: Option<&PriorShapes>,
) -> (ShapePlan, Vec<NewShape>)

The one planner behind both entry points above.

With prior == None nothing is carried, looked up or additionally hashed, so the buffered route’s plan is exactly what it was before #5407. With Some, the retained templates go in front of the batch (see ShapePlan::shared_shapes_after), and every shape the batch would emit is first looked up by content hash: a collator template as well as a stage-2 first occurrence, since a group whose template is bit-identical to an earlier batch’s shape can point at that block just as a lone repeat can.