Document infallible worker join

src/tests.rs

@@ -105,6 +105,10 @@ fn test_sendable_is_send_sync() {
 }
 
 #[test]
+#[expect(
+    clippy::expect_used,
+    reason = "the worker must not panic in this retained-element transfer test"
+)]
 fn test_send_type_erased_vec_retains_send_elements_across_threads() {
     let erased = SendTypeErasedVec::new(vec![Cell::new(42)]);