Close route off the caller thread

- Complete the async route close on the worker executor so the cross-thread assertion no longer depends on whether the first stage finished before the handler was attached

core/src/test/java/art/arcane/iris/engine/history/GenerationHistoryRuntimeRouterTest.java

@@ -723,10 +723,10 @@ public final class GenerationHistoryRuntimeRouterTest {
                     assertThrows(IllegalStateException.class, route::close);
                 }
             }, executor);
-            CompletableFuture<Void> completion = work.whenComplete((ignored, failure) -> {
+            CompletableFuture<Void> completion = work.whenCompleteAsync((ignored, failure) -> {
                 completionThread.set(Thread.currentThread());
                 route.close();
-            });
+            }, executor);
             completion.get(5L, TimeUnit.SECONDS);
         } finally {
             executor.shutdownNow();