Embed salt and KDF parameters in new envelope format

[michaelmawhinney]

Parent: #19

Problem

The current API requires callers to store and supply a separate salt. Salt is not secret, but losing it makes ciphertext undecryptable and increases misuse risk.

Tasks

• Embed salt in the new envelope format
• Store KDF algorithm identifier
• Store KDF opslimit
• Store KDF memlimit
• Ensure decrypt uses per-ciphertext KDF metadata
• Add tests for non-default KDF parameters