Function fold_v1_binding 

pub fn fold_v1_binding(
    binding_id: &str,
    mem: &str,
    v1: &LegacyBindingV1,
    configs: &PipelineConfigs,
) -> Result<Binding, BindingMigrateError>

Fold one v1 three-file-store binding into a v2 Binding: every field carries over verbatim, version becomes 2, and each source_facets entry folds its facet + medium inline under the facet’s name byte-verbatim (it keys sync watermarks). Pure — no IO.