feat(axum): add interaction registry alias

Add an interactions()/on() registration path and move canonical examples off explicit register_handle naming while keeping low-level registry types available underneath.

req: axum_integration/003

req: ceremony/001

req: dx/003

req: examples/003

slhx-axum/src/lib.rs

@@ -266,6 +266,10 @@ pub const fn handlers(fingerprint: BuildFingerprint) -> HandlerRegistry {
     HandlerRegistry::new(fingerprint)
 }
 
+pub const fn interactions(fingerprint: BuildFingerprint) -> HandlerRegistry {
+    HandlerRegistry::new(fingerprint)
+}
+
 impl InteractionRequest {
     pub fn dispatch(
         self,
@@ -320,6 +324,17 @@ impl HandlerRegistry {
         self.register(handle.id().id, handler)
     }
 
+    pub fn on<I, E>(
+        self,
+        handle: Handle<I>,
+        handler: impl Fn(InteractionForm) -> E + Send + Sync + 'static,
+    ) -> Self
+    where
+        E: IntoEffect,
+    {
+        self.register_handle(handle, handler)
+    }
+
     pub fn dispatch(&self, form: InteractionForm) -> Result<EffectResponse, DispatchRejection> {
         let handle_id = form.handle_id;
         let Some(handler) = self.handlers.get(&handle_id) else {