Function serialize_combined_for_layout 

pub(crate) fn serialize_combined_for_layout(
    meshes: &[MeshData],
    layout: ParquetLayout,
    baked_basis: Option<&Matrix4<f64>>,
) -> Result<Bytes, ParquetError>

The whole-model body of POST /api/v1/parse/parquet under the layout the client asked for, already in the outer [geo_len][geo_bytes][data_model_len=0] framing. Only SharedShapes runs the shape-sharing planner; a default request gets the pre-#3888 bytes (#3888). Framed by frame_combined_sections, which guards the outer length (it can pass u32::MAX while every section passes) and writes both frames in one allocation.