fix(identity): resolve identity assertion references into ingredient claims

[mrappard]

Changes in this pull request

1. References into ingredient claims are rejected. When reading, SignerPayload::check_against_partial_claim checks referenced assertions only against the identity assertion's own claim. A reference to an assertion in an ingredient's claim is therefore reported as cawg.identity.assertion.mismatch. CAWG Identity Assertion 1.3 allows these references:

• §5.1.1: each referenced assertion must appear "in the created_assertions, gathered_assertions … of the C2PA claim or any ingredient's claim."
• §7.1 step 2 checks "the C2PA claim or any ingredient's claim (traced recursively)."

2. cawg.identity.hard_binding_incorrect is never reported. The hard-binding check only requires some hard-binding reference. §5.1.1 requires the reference to be the claim's own hard binding, and to not reference any other. §7.1 step 4 reports cawg.identity.hard_binding_incorrect otherwise.

Changes:

• PartialClaim gets a crate-private list of ingredient-claim assertions. Manifest::from_store fills it with the assertions of the claim's ingredient claims, traced recursively, using absolute URIs (self#jumbf=/c2pa/<ingredient label>/c2pa.assertions/<label>).
• check_against_partial_claim resolves a reference against the claim first, then against the ingredient claims by exact URI. The hash is compared as before.
• A hard-binding reference that isn't in the claim itself is reported as cawg.identity.hard_binding_incorrect. This adds ValidationError::IncorrectHardBindingAssertion and validation_status::CAWG_IDENTITY_HARD_BINDING_INCORRECT. Adding a variant to ValidationError, which isn't #[non_exhaustive], is technically a breaking change for exhaustive matches.

Not changed: SignerPayload::check_against_manifest (used by IdentityAssertion::validate) and the Reader::post_validate path still build their partial claim from the claim alone.

Tests: two new fixtures, generated with mrappard/c2pa-rs-text-support cawg/conformance-fixtures. Both are CA.jpg with the CA.jpg ingredient and an X.509 identity assertion.

• identity_assertion_may_reference_ingredient_claim uses validation_method/references_ingredient_assertion.jpg. Its identity assertion also references the ingredient manifest's c2pa.actions. The test checks that no cawg.* failure is reported.
• identity_assertion_with_ingredient_hard_binding uses validation_method/hard_binding_incorrect.jpg. Its identity assertion references the ingredient manifest's c2pa.hash.data instead of its own. The test checks that the only cawg.* failure is cawg.identity.hard_binding_incorrect.

Both fail without the fix, and pass with OpenSSL and with rust_native_crypto. cargo test -p c2pa --lib --features file_io passes, and Clippy is clean.

Conformance results

Found with a CAWG identity assertion conformance suite I'm building; it's not public yet.

• identity_references_ingredient_assertion now passes.
• identity_hard_binding_incorrect now gets the right code. It fully passes together with fix(identity): do not report well-formed for an identity assertion that failed validation #2837, which stops cawg.identity.well-formed from being reported alongside the failure.

Checklist

• This PR represents a single feature, fix, or change.
• All applicable changes have been documented.
• Any TO DO items (or similar) have been entered as GitHub issues and the link to that issue has been included in a comment. (No new TODO items.)

🤖 Generated with Claude Code