gate ML‑KEM/ML‑DSA specific SHA3 APIs behind optional features (#1532)

[Arul-Sujith]

This PR gates the ML‑KEM and ML‑DSA specific SHA3 APIs behind optional Cargo features (mlkem and mldsa). The change cleans up the public API surface of libcrux-sha3 by hiding specialized incremental functions (e.g., x4::incremental, x2::incremental, and portable::incremental) unless one of the new features is enabled.

Changes

• Added mlkem and mldsa optional features to crates/algorithms/sha3/Cargo.toml.
• Guarded ML‑KEM/ML‑DSA specific modules (avx2::x4, neon::x2, portable::incremental) with #[cfg(any(feature = "mlkem", feature = "mldsa"))].
• Updated dependent crates (libcrux-ml-kem, libcrux-ml-dsa, ctgrind-test) to enable these features where needed.
• Modified unit tests and benchmarks to be conditionally compiled under the same feature gate.

Fixes #1532

[robinhundt]

Thanks for the PR! We recently added a policy on LLM-assisted submissions.

Did you use any AI tooling? If yes, to what capacity?

[Arul-Sujith]

Yes, I used an LLM to assist with this PR. Primarily to understand codebase, apply fixes, and generate PR descriptions.

Then manually review the fixes and run tests. I will make sure to include this disclosure upfront in future PR descriptions.

[robinhundt]

Is there a reason for this PR being stacked on top of #1541 ?

[Arul-Sujith]

That was an accident! I forgot to branch off of main when starting this SHA3 work, so it included the commits from my previous PR. There is no intentional dependency between them. I have rebased this branch cleanly onto main to un-stack it!