Resolves verification keys by keyid.
Key management, including JWKS fetching, caching, and rotation, is outside
this package. The callback context carries %{label:, tag:, message:, params:}
so resolvers can discriminate when key ids alone are ambiguous.
The resolved {algorithm, material}, not the message's alg parameter,
decides the verification algorithm. An alg parameter is only cross-checked
against the resolved algorithm.