Constant PLUGIN_CALLBACK_API_ID 

pub const PLUGIN_CALLBACK_API_ID: u16 = 2u16;

Wire-level API identifier for the PluginCallback service.

Default value is a 16-bit FNV-1a hash of the trait name. Since the API id space is only 2^16 wide, collisions are possible across unrelated services. For production deployments, override explicitly:

#[myelin::service(api_id = 0x0001)]
pub trait PluginCallbackService { /* ... */ }