Function prune_guarantee_for_medium 

pub fn prune_guarantee_for_medium(medium_type: MediumType) -> PruneGuarantee

The strongest prune guarantee a medium can support (F1), derived from the capability matrix: a base-leg-retrievable medium (git-backed — codebase / filesystem / git / graph) supports the full never-clobber three-way merge; a non-retrievable medium (web) supports only conflict-flag degradation. Validation refuses a request that exceeds this.