2e41e0bae0
Promotes ParamsFile to a module-level pub(crate) struct with both Serialize and Deserialize derives. for_new_vault() constructor + into_kdf_params() inversion replace the two-definition split between commands/init.rs (write) and session.rs read_params (read). On-disk JSON format unchanged — fixture test asserts round-trip with the current params.json layout.