CVE-2025-2886: tough terminating targets role delegations are not respected

Delegations are a mechanism defined by the TUF specification that allow multiple different identities to provide and sign content within a single repository. Terminating delegations and delegation priority give a TUF repository unambiguous control over how overlapping delegations are resolved. tough erroneously will not terminate a search as required, and will accept information from a lower-priority delegation that should have been ignored.