Draft: WIP: Remember the parent home unit during linking
requested to merge wip/gkaracha/mercury-mhu-transitive-th-deps into wip/torsten.schmits/mercury-ghc9101-backport-2025-07-09
Part of https://github.com/tweag/mercury-ghc-internal/issues/159. Just parking this here for visibility - it's not to be merged anytime soon.