Macro register_payload_protocol 

macro_rules! register_payload_protocol {
    ($(#[$meta:meta])* $vis:vis const $name:ident: u32 = $value:expr;) => { ... };
}

Define a consumer payload-protocol constant with compile-time range and first-party-collision checks.

The authoritative allocations remain in registry; this macro verifies only that a consumer’s pinned literal belongs to an allowed allocation range and cannot collide with a first-party protocol.