feat(axum): build forms from checked handles

Add InteractionForm::for_handle so tests and public integration code can construct dispatched forms from generated Handle<T> values instead of raw numeric handle ids.

req: ceremony/004

req: public_api/001

slhx-axum/tests/response.rs

@@ -109,7 +109,7 @@ fn handler_registry_dispatches_by_checked_handle() {
     });
 
     let response = registry
-        .dispatch(InteractionForm::new(create.id().id, [("title".into(), "Hello".into())]))
+        .dispatch(InteractionForm::for_handle(create, [("title".into(), "Hello".into())]))
         .unwrap();
 
     assert_eq!(response.batch.fingerprint, BuildFingerprint(123));