citizen: implement Convert latest review findings into one concrete code change with a short validation note.

contracts/LabHelper.sol

@@ -0,0 +1,14 @@
+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.24;
+
+contract Main {
+    string public topic;
+
+    constructor(string memory initialTopic) {
+        topic = initialTopic;
+    }
+
+    function setTopic(string calldata nextTopic) external {
+        topic = nextTopic;
+    }
+}