refactor(v0): name generated interaction handlers

Expose and use InteractionHandlers in the beginner example so the normal path reads as app handlers instead of registry plumbing while preserving the integration registry API.

req: ceremony/005

hemx-axum/src/lib.rs

@@ -225,6 +225,7 @@ pub struct HandlerRegistry {
 }
 
 pub type Registry = HandlerRegistry;
+pub type InteractionHandlers = HandlerRegistry;
 
 pub struct StateHandlerRegistry<S> {
     registry: HandlerRegistry,