pub const PROTOCOL_VERSION: u32 = 3;Expand description
Separate from RPC_PROTOCOL_VERSION; bump on any non-additive change.
v2 (phase 3): Set became a struct variant, Bye gained a reason.
v3 (E-P0): driver events (DriverArmed, DriverRevoked,
DriverTurnOutcome); old clients skip unknown via #[serde(other)].
Protocol stays exact-match; only binary versions compare directionally
(reload, §2.8).