Prepare SAFE 0.4.0 release

[HDauven]

Summary

Prepare dusk-safe 0.4.0, stacked on #41.

• Bump the package version from 0.3.0 to 0.4.0.
• Move the accumulated Unreleased entries into the dated 0.4.0 section.
• Keep an empty Unreleased section and update the release comparison links.

This PR changes only Cargo.toml and CHANGELOG.md: +5/-2 lines relative to #41. It introduces no additional runtime, API, dependency or test changes.

Compatibility

0.4.0 is the intended breaking release for the removal of Sponge: PartialEq in #40, tracked in #39. Consumers using whole-sponge equality or requiring that trait must update their code; the migration is documented on Sponge. The changelog also records the Rust 1.96.1 MSRV and the accumulated hardening fixes.

Validation

Passed locally on Rust 1.96.1:

• make test and make cq, including debug/release tests and the zeroize 1.5.4 compatibility consumer.
• No-default-features tests, and thumbv6m-none-eabi builds with and without encryption.
• Documentation and all four frozen AE mechanics vectors.
• cargo package --all-features and cargo publish --dry-run --all-features.
• Debug/release, no-default-features, no_std, documentation and fixture checks repeated from the extracted 0.4.0 package.

The verified package contains 22 files and records commit 48db4a9cf975a4d3e63f54b43c4d78a8df22c512 with a clean working tree.

Merge / publication

Merge #41 before this PR. This PR currently targets fix/absorb-validated-input so its incremental diff is limited to release metadata.

This prepares the release only: no tag or publication has been performed. Before publishing, verify the final merged release commit and its checks; refresh the changelog date if publication happens on a later date.