Deferred: Return matched step delta from TOTP verification for clock-drift detection

[darianmiller]

Follow-on from #14. #14 implemented TTOTP.ValidatePassword returning Boolean
over a +/- N step window. RFC 6238 section 6 and RFC 4226 resynchronization
describe using the matched step offset to detect and correct a client's
persistent clock drift, which the current Boolean API discards.

Deferred scope:

• Add a verification overload that reports the matched step delta (e.g. an out
  parameter or a small result record { Matched: Boolean; StepDelta: Integer }),
  where StepDelta is the offset from the reference step at which the candidate
  matched (0 = exact, -1 = one step behind, +1 = one step ahead).
• A server can persist per-account drift and pre-shift the reference step on
  subsequent validations.

Implementation notes:

• Builds on ValidateAtCounter in radRTL.TOTP. The loop already visits each step
  in the window; capture the delta on match without adding an early-out.
• Keep the existing Boolean ValidatePassword overloads. The delta is returned to
  the (trusted) caller; the full window is still evaluated so timing does not leak
  which step matched.

Acceptance criteria:

• A verification overload reports whether the candidate matched and at what step delta.
• StepDelta is 0 for an exact match, negative for earlier steps, positive for later steps.
• The full window is still evaluated (no early exit) to preserve constant-time behavior.
• Tests cover exact, -1, +1, and no-match cases via the deterministic (explicit-time) overload.