Pass SmartShift parameters as a typed struct at the next PROTOCOL_VERSION bump

`Agent::set_smartshift(route, mode: SmartShiftMode, auto_disengage: u8, tunable_torque: u8)` threads three loose values through five layers, where `0` is a magic "do not change" sentinel documented only at the bottom (`write/smartshift.rs`). The right encoding already exists — hidpp's `SmartShiftEnhancedStatusChange` uses `Option`.

Fold the trio into a Change/Params struct end to end. The tarpc method signature is append-only wire format, so this must ride the next `PROTOCOL_VERSION` bump (regenerate the `wire_format.rs` goldens) rather than ship alone.

Surfaced by the standards audit behind #381.

Trace Agent::set_smartshift and the three values through the five layers, starting with write/smartshift.rs and the existing hidpp::SmartShiftEnhancedStatusChange encoding. Review the tarpc method signature and wire_format.rs goldens before the next PROTOCOL_VERSION bump. Done means the values travel as a typed Change/Params struct, the wire-format goldens are regenerated, and the sentinel semantics are preserved.

Written by the indexing model from the issue text.