idz: Hoist userdb code

src/idz/userdb/decoder/saveStocker.ts

@@ -0,0 +1,19 @@
+import { bitmap } from "./_bitmap";
+import { chara } from "./_chara";
+import { CarSelector } from "../model/car";
+import { SaveStockerRequest } from "../request/saveStocker";
+import { AimeId } from "../../../model";
+
+saveStocker.msgCode = 0x00a6;
+saveStocker.msgLen = 0x00c0;
+
+export function saveStocker(buf: Buffer): SaveStockerRequest {
+  return {
+    type: "save_stocker_req",
+    aimeId: buf.readUInt32LE(0x0004) as AimeId,
+
+    backgrounds: bitmap(buf.slice(0x0008, 0x002c)),
+    selectedCar: buf.readUInt16LE(0x009c) as CarSelector,
+    chara: chara(buf.slice(0x009e, 0x00b2)),
+  };
+}