DustBackupEncryptor

AES-256-GCM encryption for the dust cloud-backup bundle.

Same construction as AppStateBackupEncryptor (version || IV(12) || ciphertext+GCMtag) but size-agnostic: the dust state is ~500 KB, far past that encryptor's 2 KB metadata cap, and the bundle (DustCloudBundleCodec) carries its own internal lengths — so this encryptor just seals/opens an arbitrary byte payload with no length prefix or cap. Kept separate so the load-bearing passkey-backup encryptor is untouched.

Throws BackupDecryptionException (shared with AppStateBackupEncryptor) on a malformed blob or AEAD verification failure.