Mirror of hanzo-engine/src/poi_transcript.rs: ProofTranscript (keccak Merkle over per-matmul
leaves), ExactMatmul (whole-K i64), VerifyOpening (Merkle inclusion + Freivalds), ChallengeIndex.
VerifyOpening is the production caller the audit found missing — the off-chain watcher imports it
and slashes a bond when an opening fails. Tests: transcript round-trip, fabricated-output-caught,
swapped-reveal-caught, and a pinned golden DeriveChallenges vector mirrored byte-for-byte in the
Rust suite (cross-language parity enforced, not assumed). go test ./poi: ok.
