[HDauven]

Summary

• Remove PartialEq from Sponge, retaining Clone, so the API no longer exposes ordinary whole-state equality over secret state and buffered output.
• Add migration guidance and a compile-fail regression using equality-capable backend/element bounds.
• Remove the stored domain_sep field, which becomes unused without derived equality. The constructor still includes the domain separator in the initial tag.

Fixes #39.

Compatibility

Breaking Rust API change for the next breaking release. Remove whole-sponge comparisons and Sponge: PartialEq requirements when upgrading; no replacement comparison API is introduced. This PR does not bump the package version.

Sponge state width, arithmetic, pattern encoding, field-based zeroization and backend authentication through Encryption::is_equal are unchanged. The inspected Poseidon callers keep sponges local and return outputs rather than exposing a serialized or FFI sponge layout. This is API hardening, not a claim of a demonstrated deployed timing leak.

Validation

Rust 1.96.1, final source, local checks:

• make test: 20 debug + 20 release test executions; existing zeroize 1.5.4 consumer check passes.
• No-default-feature tests: 13 pass.
• make cq, documentation, and bare-metal builds with/without encryption pass.
• Existing frozen encryption fixtures match; no fixture was regenerated.
• Downstream release tests: Poseidon 20, Phoenix-core 29, Poseidon-Merkle 2, Rusk-core 65 pass with this SAFE source selected by Cargo patches. These are the selected existing consumer snapshots, not every downstream version or a full-node replay.
• Negative control: restoring only PartialEq to the final source makes the new doctest fail because the supposedly non-compiling comparison compiles successfully.

Only two files ship; benchmark and investigation scaffolding remains local. No contract source, WASM, dependency or CI change is included.