Absorb elements from the validated input slice

[HDauven]

Summary

Borrow the input once in Sponge::absorb, so the slice checked for sufficient length is also the slice that is absorbed.

Previously, the two as_ref() calls could return different slices. A safe Rust adapter that returns two elements on its first call and an empty slice on the second makes absorb(2, ...) report success without absorbing either element.

This behavior dates to SAFE's initial implementation; it was not introduced by #40. This PR is based directly on current main, after #40.

Changes

• Reuse the validated slice throughout absorption.
• Add one regression using the existing Rotate test backend.
• Add an Unreleased changelog entry.

No new API, dependencies, allocations or changes to field/permutation logic. Ordinary slice/array inputs retain their existing behavior. The diff is three files, +23/-2 lines.

Validation

• The exact regression fails on unmodified main in debug and release (expected 9, actual 0), and passes with the fix.
• make test, make cq, no-default-features tests, no_std builds with and without encryption, docs and frozen-vector checks pass.
• Selected local Poseidon, Phoenix-core, Merkle and Dusk-core test suites pass with this SAFE candidate patched in.
• Additional local checks passed: one million seeded model inputs in each of debug, release and 32-bit execution; 1.54 million coverage-guided executions; allocation/cleanup tests and focused Miri checks. Selected deliberately faulty implementations were detected.

Investigation harnesses, fuzz corpora and mutation tests are intentionally outside this PR. These are bounded correctness checks, not a proof of cryptographic security; no deployed impact or authentication bypass was demonstrated.

[Copilot]

🟢 Approval recommended

No unresolved review issues remain.

Pull request overview

Fixes Sponge::absorb by reusing the slice validated for length, preventing stateful AsRef inputs from being inconsistently handled.

Changes:

• Reuse the validated input slice.
• Add a regression test for stateful AsRef inputs.
• Document the fix in the changelog.

File summaries

File	Description
tests/sponge.rs	Adds regression coverage.
src/sponge.rs	Reuses the validated input slice.
CHANGELOG.md	Records the fix.

Review details

• Files reviewed: 3/3 changed files
• Comments generated: 0
• Review effort level: Lite

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