fix: add configurable zero-dollar proof replay protection

[brendanjryan]

Summary

• add optional store-backed replay protection for zero-dollar proof credentials keyed by challenge id
• keep proof replay prevention configurable: without an explicit store, proofs remain reusable until expiry
• add regression coverage for both configured-store and no-store zero-dollar auth flows

[pkg-pr-new]

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npm i https://pkg.pr.new/mppx@268

commit: 851ac5f