Remove secret-state equality from Sponge in the next breaking release #39

Follow-up from #38

The pre-existing #[derive(Clone, PartialEq)] on Sponge includes its permutation state and buffered output. Ordinary derived equality provides no constant-time guarantee. Track this as a separate API-hardening and compatibility decision, as suggested in moCello's review.

This is not a confirmed deployed timing leak or authentication bypass: no attacker-observable caller comparing whole sponge instances has been identified. Encryption's authentication check separately uses the backend's Encryption::is_equal contract; removing the derived trait does not replace that obligation.

Proposed scope

• Check downstream consumers for actual use of Sponge equality.
• Prefer removing PartialEq in the next breaking release, with a migration/changelog note. Do not silently redefine whole-sponge equality to compare metadata only.
• If a comparison API is genuinely needed, specify its purpose and constant-time/backend requirements explicitly before adding a replacement.
• Keep the sponge algorithm, valid encoding and cryptographic outputs unchanged.

Source reviewed: Sponge at bf63c009 and authentication comparison.

Related: #37. This follow-up is intentionally outside #38's merge scope.