fix(verify): handle canonical Rekor intoto entries

[wolfv]

Summary

• model Rekor v1 intoto/0.0.2 canonical entries using payloadHash, payloadType, envelope hash, signatures, and signature public keys instead of requiring the proposed DSSE payload
• bind the canonical entry payload hash and payload type to the bundle DSSE envelope
• bind every logged signature and its certificate/public key to the bundle signature and signing certificate
• recognize intoto/0.0.2 as a Rekor v1 integrated-time source rather than a Rekor v2 entry
• support SHA-512 in-toto subjects so the existing real sigstore.js@2.0.0 provenance fixture exercises complete verification
• add positive and tampering tests for payload hash, payload type, signature, and signing certificate bindings

This addresses TOB-SIGSTORE-12.

Regression coverage

Before the model change, the real canonical fixture failed while parsing its Rekor body with missing field payload. The complete verification flow now succeeds with that fixture.

Testing

• cargo test -p sigstore-rekor -p sigstore-verify
• cargo clippy -p sigstore-rekor -p sigstore-verify --all-targets -- -D warnings
• cargo fmt --all -- --check
• git diff --check

[jku]

I will rubber stamp this but I wonder if we could just drop the intoto support and focus on hashedrekord and dsse?

Anyway, lgtm

[wolfv]

I looked at this alongside the Rekor v2 work. The unfortunate naming is that intoto/0.0.2 is still a legacy Rekor v1 entry type; Rekor v2 DSSE submission is hashedrekord/0.0.2. #176 removes the nonexistent dsse/0.0.2 v2 path, but it does not make this fix obsolete. The regression fixture is a real sigstore.js provenance bundle that otherwise cannot be verified, so I think we should retain read/verification compatibility for historical intoto entries while not implementing an intoto v2 write path.

[wolfv]

@jku I checked this against the Rekor v2 work: intoto/0.0.2 is unfortunately a legacy Rekor v1 entry type; Rekor v2 DSSE submission is hashedrekord/0.0.2. So removing the nonexistent v2 dsse/0.0.2 path does not make this obsolete. The fixture is a real historical sigstore.js provenance bundle, so I think read/verification compatibility is worth keeping. I also pushed managed-key binding so an intoto verifier must match the caller-supplied key.

[tnytown]

looks like this behaves correctly, but i have a few code quality nits