Implement confidential payment settlement using Pedersen commitments

[Kingsman-99]

Overview

Allow payers to hide payment amounts on-chain using Pedersen commitments. The contract stores commitments during payment and verifies the opening (value + blinding factor) at settlement time. Only after settlement are the actual amounts visible on-chain.

Requirements

• Extend pay to accept an optional commitment: Option<BytesN<32>> — if set, store the commitment instead of the raw amount
• reveal_payment(env, invoice_id, payer, value: i128, blinding: BytesN<32>) — verifies the Pedersen commitment C = value*G + blinding*H and credits the payment
• Use a fixed generator point H stored in contract instance storage (set at init)
• Reject reveals where the commitment doesn't match
• Emit ConfidentialPaymentRevealed { invoice_id, payer } (no amount in event — privacy preserved)
• Write tests: commit → reveal succeeds, tampered value rejected, tampered blinding rejected, double-reveal rejected

Acceptance Criteria

• Commitment stored during pay (amount hidden)
• reveal_payment verifies commitment correctly
• Tampered reveals rejected
• Double-reveal rejected
• No amount leaked in events
• All existing tests pass
• cargo test --workspace passes with zero failures
• cargo clippy --all-targets -- -D warnings produces zero warnings

Definition of Done

All CI checks must pass before the PR is reviewed.

[nurudeenheroic]

Hi! I'd love to work on this. I'm confident I can deliver a clean solution.