feat(axum): start stateful interactions tersely

Add state_interactions as a small helper for app-owned stateful registries, and use it in the workout exemplar to remove one layer of repetitive route wiring without hiding generated handles or app handlers.

req: axum_integration/003

req: ceremony/001

req: dx/003

req: codegen/002

req: examples/001

hemx-axum/tests/response.rs

@@ -232,6 +232,47 @@ fn interaction_request_dispatches_with_concise_handlers_helper() {
     assert_eq!(response.batch.ops, vec![Slot::<String>::new(3).text("ok")]);
 }
 
+#[test]
+fn state_interactions_starts_stateful_wiring_without_nested_closures() {
+    // req: axum_integration/003 req: ceremony/001 req: dx/003
+    fn ping(prefix: String) -> impl IntoEffect {
+        Slot::<String>::new(3).text(format!("{prefix}: ping"))
+    }
+
+    fn open(prefix: String, input: OpenProject) -> impl IntoEffect {
+        Slot::<String>::new(3).text(format!("{prefix}: {}", input.project_id.0))
+    }
+
+    let registry = || {
+        hemx_axum::state_interactions(BuildFingerprint(4), "project".to_owned())
+            .on_state(Handle::<()>::new(7), ping)
+            .on(Handle::<()>::new(8), open)
+            .into_registry()
+    };
+
+    let ping_response = InteractionRequest::from(InteractionForm::for_handle(
+        Handle::<()>::new(7),
+        Vec::new(),
+    ))
+    .dispatch(registry())
+    .unwrap();
+    assert_eq!(
+        ping_response.batch.ops,
+        vec![Slot::<String>::new(3).text("project: ping")]
+    );
+
+    let open_response = InteractionRequest::from(InteractionForm::for_handle(
+        Handle::<()>::new(8),
+        vec![("project_id".to_owned(), "42".to_owned())],
+    ))
+    .dispatch(registry())
+    .unwrap();
+    assert_eq!(
+        open_response.batch.ops,
+        vec![Slot::<String>::new(3).text("project: 42")]
+    );
+}
+
 #[test]
 fn interaction_request_dispatches_typed_form_inputs() {
     // req: axum_integration/003 req: form/004 req: canonical_authoring/003