chore: remove inappropriate tests from tarpaulin runs

tests/arc_tests.rs

@@ -1092,6 +1092,7 @@ async fn test_arc_mixed_with_non_arc() {
 
 /// Test 16: Arc in parallel execution paths
 #[tokio::test]
+#[cfg_attr(tarpaulin, ignore)] // Timing tests are unreliable with instrumentation overhead
 async fn test_arc_parallel_execution() {
     struct SharedProducer;