Propose and validate mappings

Each proposal has kind, from, and to; both endpoints must use the same kind. The supplied fixture explicitly proposes Custom C → D: C exists in the previous manifest and D is persistent in the current declarations. SaveTether never guesses a mapping from similar names, identifiers, asset paths, or files.

Proposals may form chains, but a chain must terminate at a persistent identity in the current declarations. Sources must be unique. The finding SAVETETHER-MAPPING-REJECTED includes a reason:

• Cycle: proposals form a loop, including a self-map.
• DuplicateMappingSource: the same typed source is proposed more than once.
• MissingPreviousSource: the source is absent from the selected prior release.
• MissingDestination: the chain does not end at a current persistent identity.

Cross-kind endpoints are invalid as well. An accepted proposal records reviewed intent only; it does not rename an identity, alter declarations, install redirects, or migrate saves. Any rejected proposal blocks release capture. Correct the declaration, run a new review, and verify the finding is gone.