[On-Chain] Optimize PlonK on-chain verifier (PlonkBLS12381Lib) CPU budget on Plutus V3

[satran004]

Background & Finding (SEC-10)

In zeroj-onchain-julc, the deployable PlonK verifier (PlonkBLS12381Lib.java) performs a full linearized polynomial commitment evaluation and batched KZG pairing check on-chain.

Currently, PlonK verification on Plutus V3 consumes ~4.8–4.95e9 CPU units (~49% of the transaction block budget limit), compared to ~2.14e9 CPU units (~21%) for Groth16. Reducing PlonK verification costs improves transaction composability on Cardano.

Scope & Tasks

• Profile Plutus Core step-by-step evaluation inside PlonkBLS12381Lib.
• Optimize linearized polynomial evaluation, scalar multiplications, and Fiat-Shamir hash invocations in Plutus Core.
• Target reducing CPU budget from ~4.9e9 down toward ~2.5e9.
• Validate on-chain execution against Yaci DevKit.

Risk Classification

• R2: Cardano on-chain verifier performance & feasibility.
• Reference: docs/zeroj-comprehensive-review-and-readiness-gemini.md (SEC-10, Section 5.4).