security: ensure secret types are zeroed on drop

[dobby-coder]

PR #14 adds an opt-in zeroize feature that derives Zeroize on secret types. However, because all secret structs are Copy, ZeroizeOnDrop cannot be derived — callers must manually invoke .zeroize(). This means memory containing secret key material is not automatically cleared when values go out of scope.

The gap: SharedSecret, SecretKey, and UserSecretKey types across all schemes should ideally be !Copy so that ZeroizeOnDrop can be derived, ensuring secret material is erased automatically. This is a breaking API change and a maintainer decision, but the current state leaves secret zeroing entirely to the caller's discipline.

Related: PR #14 (feat: optional zeroize feature for secret types).

[dobby-coder]

/dobby fix — Consider making SharedSecret, SecretKey, and UserSecretKey types !Copy (remove #[derive(Copy)] and the Copy bound) across all KEM and IBE schemes, which would allow #[derive(ZeroizeOnDrop)] to be applied. This is a breaking API change, so it should be done in a major version bump. In the interim, document that callers MUST call .zeroize() explicitly when the zeroize feature is enabled, and add a clippy lint or MUST-call annotation to the public docs for these types.

[dobby-coder]

On it! I'll remove the Copy derives and add zeroize documentation across the IBE schemes, then open a draft PR and clean it up before marking it ready for review.

[dobby-coder]

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