Add debug mode and VM tracing

Introduce a `debug_mode` flag to the `Environment` and a new `run_debug`
method. This method uses a `TracingObserver` to log the VM's execution
flow, including node evaluation and scope state changes. This allows for
detailed inspection of script execution.

The `BoundKind` enum now also includes a `display_name` method for
better log readability.

src/integration_test.rs

@@ -120,4 +120,31 @@ mod tests {
             }
         }
     }
+
+    #[test]
+    fn test_debug_mode_logging() {
+        let env = Environment::new();
+        let source = "(+ 10 20)";
+        let result = env.run_debug(source).expect("Failed to run debug");
+        
+        let (val, logs) = result;
+        
+        // 1. Check value
+        match val {
+            Ok(Value::Int(30)) => (),
+            _ => panic!("Expected Int(30), got {:?}", val),
+        }
+
+        // 2. Check logs (should have entries for + and constants)
+        assert!(!logs.is_empty(), "Logs should not be empty");
+        
+        // Look for typical trace patterns
+        let has_call = logs.iter().any(|l| l.contains("CALL"));
+        let has_const = logs.iter().any(|l| l.contains("CONST(10)"));
+        let has_result = logs.iter().any(|l| l.contains("} -> 30"));
+
+        assert!(has_call, "Logs should contain CALL");
+        assert!(has_const, "Logs should contain CONST(10)");
+        assert!(has_result, "Logs should contain result 30");
+    }
 }