Expose DataProofs' context allowlisting (ProofVerificationOptions.KnownContexts) through CredentialVerificationOptions #22

Context

DataProofs 2.0.0 added ProofVerificationOptions.KnownContexts: context URI allowlisting for Data Integrity verification. A proof over RDF binds the statements, not the JSON that compacted them, so a holder can present a verified document under a @context that reads its terms differently (the upstream example: boards read as sails), and it verifies on every conforming implementation. VC Data Integrity §4.6 / §2.4.1 therefore require applications to accept only known contexts. With KnownContexts set, every @context value in the secured document, at any depth, must be a listed IRI; an unlisted value, an inline context object or a null fails the document with a document-level PROOF_VERIFICATION_ERROR. Unset (today's behaviour), nothing is checked.

Credentials passes ExpectedProofPurpose / ExpectedChallenge / ExpectedDomain to the pipeline (src/Credentials.Core/Securing/Internal/DataIntegrityMechanism.cs) but does not expose KnownContexts, so a Credentials verifier cannot enable this protection without bypassing IVerifier.

Proposal

• Add IReadOnlyCollection<string>? KnownContexts to CredentialVerificationOptions (and thread it through PresentationVerificationOptions / contained-credential options and the status-list / schema-credential recursion), mapped to ProofVerificationOptions.KnownContexts for the embedded Data Integrity form. Null keeps today's behaviour.
• Decide whether the enveloping forms (JOSE / COSE / SD-JWT) should apply the same allowlist to the inner document's @context for consistency; they sign the JSON itself, so the respelling attack does not apply to them, but a uniform policy surface may still be wanted.
• Document (SECURITY.md "Best practices") that an RDFC/bbs-2023 verifier acting on claim semantics should set it, with approved immutable contexts, as upstream advises: listing a context trusts what it references, and the option does not authenticate context contents.
• Tests: an RDFC credential re-compacted under a foreign context is Rejected when the list is set and Accepted when unset; nested contexts and inline context objects; presentations embedding credentials with their own contexts.

This is a public API addition (additive, non-breaking; PublicAPI.Unshipped.txt + ApiCompat), so it did not belong in the dependency-update PR (#21), where it was noted as a follow-up.

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