pqc_kyber: AVX2 backend skips Fujisaki-Okamoto implicit rejection, enabling chosen-ciphertext key recovery

Summary

## Summary

When `pqc_kyber` is built with the opt-in `avx2` feature on x86_64, its constant-time conditional-move routine `cmov` is a no-op: it never performs the copy. That `cmov` is exactly the step that applies the Fujisaki-Okamoto (FO) implicit rejection during decapsulation. With it disabled, decapsulating an *invalid* ciphertext no longer returns a key-independent pseudorandom value; it returns a value that depends on the decrypted plaintext of the attacker-chosen ciphertext. This restores the chosen-ciphertext decryption oracle that the FO transform exists to remove, and yields full recovery of the static secret key.

The attack requires no timing measurement, no side channel, and no faults; only the ability to submit ciphertexts to a decapsulation operation under a reused key pair and observe the resulting shared secret. See the linked pull request for the root cause, the reachable call path, and reproduction details.

## Affected configuration

Reachable only in builds with `features = ["avx2"]` on an x86_64 target. The default (reference) backend and all non-x86_64 targets are unaffected.

## Impact

Complete IND-CCA break. The plaintext-checking oracle drives a standard chosen-ciphertext key-recovery attack and extracts the entire static secret key. Any protocol reusing a Kyber key pair across decapsulations (static or long-term KEM keys, KEMTLS, HPKE recipients, pinned keys) is exposed; ephemeral-only key shares are not. The defect was verified end to end on ML-KEM-768: the full secret key was recovered in 4,272 decapsulation queries.

## Maintenance status

The crate is unmaintained (last release 0.7.1, August 2023); no patched version exists or is expected.

## Mitigation

- If remaining on this crate, do not enable the `avx2` feature; the default reference backend performs implicit rejection correctly. - Do not reuse a Kyber key pair across decapsulations.

## Credit

Reported by 007bsd.