totp_verify.nx

totp_verify.nx -- TOTP code verification with clock-skew window. Companion to totp_sha1.nx. A bare "is the code equal to totp(secret, now)?" check fails whenever the user's device clock drifts by a few seconds past the step boundary. RFC 6238 §5.2 recommends accepting the code from step N-1 and N+1 as well, for a 3-window tolerance. We generalise to caller-chosen window size. Uses constant-time comparison (ct.nx) across all candidate windows so timing doesn't leak which window matched (hardening against code-enumeration attacks from a malicious client). Invariants: TV1 Provided code compared against every window in [now_step - window, now_step + window], inclusive. TV2 Match check uses ct_eq on the integer value -- no timing side-channel per window. TV3 Returns matched window offset (negative meaning older step, 0 = current, positive = newer) on success, or TV_ERR_NO_MATCH if nothing in range matches.