@btx-tools SDK — BTX service-challenge admission control
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    VerifyReason:
        | "ok"
        | "invalid_proof"
        | "challenge_mismatch"
        | "expired"
        | "unknown_challenge"
        | "already_redeemed"
        | "missing_proof"
        | string & {}

    Reasons returned by verify/redeem. Open string — btxd may add more.