Harden strace benchmark binaries

Make the standalone protocol operation examples return and print concrete checksums derived from protocol state so the traced work cannot be optimized away as dead computation.

examples/protocol_op_forward_call_receive.rs

@@ -3,6 +3,6 @@ mod common;
 
 fn main() {
     let iterations = common::iterations_from_args(1_000);
-    common::run_forward_call_receive(iterations);
-    println!("forward_call_receive iterations={iterations}");
+    let checksum = common::run_forward_call_receive(iterations);
+    println!("forward_call_receive iterations={iterations} checksum={checksum}");
 }