fix(paillier): stop caching private-key decryption state; review follow-ups

[piotr-roslaniec]

Follow-ups to the review fixes pushed directly to dev (873b8ad..fa6ef4d), per decisions recorded on #37.

Changes

• Drop the private-key Paillier cache (crypto/paillier/ct_cache.go, PrivateKey.Decrypt). The cache kept copies of LambdaN, its byte encoding, and the decryption coefficient in a package-global map for the key's lifetime and never zeroed them. It measured no Decrypt gain on a reused 2048-bit key (median 70.7 ms before vs 70.6 ms after the cache). Decrypt rebuilds its state per call again; ExpCTWithBitLen wipes its padded exponent. Kept: up-front key validation, ErrInvalidModulus/ErrMalformedKey error returns in both timing modes, and the public-key N^2 cache (Encrypt ~10% faster, half the allocations).
• CI: go directive check. The Vet job fails if go.mod's go directive differs from the minimum stated in README. It would have caught test: add holistic signing boundary regressions #33 lowering the directive to 1.25.6.
• Changelog: the review fixes were credited to "(PR Integrate dev hardening set into master #37)", the dev->master tracking PR. They are now attributed to their commits.

Verification

• go test -shuffle=on ./crypto/paillier ./crypto/mta: pass
• go test -race -count=3 on concurrent same-key, key-mutation, by-value-copy, and malformed-key Decrypt tests: pass
• go vet ./crypto/paillier: pass
• go directive check run locally: passes on current files, fails when go.mod is set to 1.25.6