idz: Hoist userdb code

src/idz/userdb/decoder/saveTopic.ts

@@ -0,0 +1,13 @@
+import { SaveTopicRequest } from "../request/saveTopic";
+
+saveTopic.msgCode = 0x009a;
+saveTopic.msgLen = 0x0010;
+
+export function saveTopic(buf: Buffer): SaveTopicRequest {
+  const aimeId = buf.readUInt32LE(0x0004);
+
+  return {
+    type: "save_topic_req",
+    aimeId: aimeId !== 0xffffffff ? aimeId : undefined,
+  };
+}