test: include Path-B fixtures in Go/Rust verification; update audit doc

[mdheller]

Motivation

• Close the parity audit finding that Path‑B (ternary) fixtures were not covered by Go/Rust verification so repack determinism was unchecked.
• Ensure full-frame deterministic verification covers both Path‑A and Path‑B fixture vectors so fixtures remain the single source of truth.
• Keep changes minimal and non-invasive by only extending test vectors and updating the audit record.

Description

• Added Path‑B fixture entries to the Go test set in go/tritrpcv1/fixtures_test.go (added fixtures/vectors_hex_pathB.txt + .nonces to the sets list around L73-L77).
• Added Path‑B fixture entries to the Rust test set in rust/tritrpc_v1/tests/fixtures.rs (added fixtures/vectors_hex_pathB.txt + .nonces to the sets list around L73-L76).
• Updated the parity audit document docs/audit_tritrpc_v1_parity.md to record and mark the Path‑B coverage gap (A8) as fixed and set the report status to completed (post‑fix).
• No runtime or public API changes; this change strictly increases test coverage and documentation alignment.

Testing

• No automated tests were executed as part of this change in the current environment.
• The repository provides make verify and per-language commands for CI/local verification (cargo test, go test, and python tools/verify_fixtures_strict.py) which will run the added fixture checks when invoked.
• CI is expected to run the full verification suite and will catch any remaining mismatches for Path‑B vectors.
• If you want to reproduce locally, run make verify from the repo root (see docs/integration_readiness_checklist.md).

---

Codex Task