feat(shared): add DDD use case contracts

src/Shared/DDD/UseCases/Executor.php

@@ -0,0 +1,10 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Shared\DDD\UseCases;
+
+interface Executor
+{
+    public function execute(): object;
+}

src/Shared/DDD/UseCases/Presenter.php

@@ -0,0 +1,19 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Shared\DDD\UseCases;
+
+/**
+ * @template TOutput of Output
+ *
+ * @template-covariant T of object
+ */
+interface Presenter
+{
+    /**
+     * @param  TOutput  $output
+     * @return T
+     */
+    public function present(Output $output): object;
+}

src/Shared/DDD/UseCases/Transactional.php

@@ -0,0 +1,13 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Shared\DDD\UseCases;
+
+/**
+ * Marker interface for use cases that perform database mutations
+ * requiring an atomic database transaction.
+ */
+interface Transactional
+{
+}