chore: address code quality audit findings

src/types/game.ts

@@ -25,6 +25,28 @@ export const GAME_STEPS: readonly GameStep[] = [
   "outOfFabrik",
 ] as const;
 
+const GAME_STEP_VALUES: ReadonlySet<string> = new Set(GAME_STEPS);
+
+export type MainGameState = "intro" | "bike" | "pylone" | "ferme" | "outro";
+
+export const MAIN_GAME_STATES: readonly MainGameState[] = [
+  "intro",
+  "bike",
+  "pylone",
+  "ferme",
+  "outro",
+] as const;
+
+const MAIN_GAME_STATE_VALUES: ReadonlySet<string> = new Set(MAIN_GAME_STATES);
+
+export function isGameStep(value: unknown): value is GameStep {
+  return typeof value === "string" && GAME_STEP_VALUES.has(value);
+}
+
+export function isMainGameState(value: unknown): value is MainGameState {
+  return typeof value === "string" && MAIN_GAME_STATE_VALUES.has(value);
+}
+
 export interface Zone {
   id: string;
   position: Vector3Tuple;