Extend strict fixture verifier and Rust/Go parity checks for Prophet evidence.v1

[mdheller]

Summary

Extend the stable v1 verifier/test surfaces to include the Prophet evidence.v1 lane fixtures.

Depends on #50.

Proposed changes

• Add vectors_hex_prophet_lane1.txt and vectors_hex_prophet_lane1.txt.nonces to the strict fixture verification set.
• Add Rust repack/parity coverage for the lane-1 vectors.
• Add Go repack/parity coverage for the lane-1 vectors.
• Ensure the verifier uses the same stable v1 rule: XChaCha20-Poly1305 tag over AAD = envelope bytes up to the final tag field length prefix.

Acceptance criteria

• python tools/verify_fixtures_strict.py checks lane-1 fixtures.
• cargo test -p tritrpc_v1 includes lane-1 vectors.
• cd go/tritrpcv1 && go test includes lane-1 vectors.
• CI passes with the new fixture file included.

Boundary rule

This issue is transport/conformance only. It should not redefine Prophet evidence semantics.

[mdheller]

Refinement after repo inspection: extending Rust/Go parity checks for Prophet lane-1 fixtures cannot simply reuse the existing HGRequest/HGResponse decode paths. Current tests assume .REQ and .RSP payloads are hypergraph types. For evidence.v1, we should either:

1. Add a parallel test path that only verifies envelope integrity + AEAD tag for non-HG payloads, or
2. Introduce a generic typed-payload decode hook in the test harness.

This issue should track that design choice before wiring lane-1 fixtures into parity tests.