feat(casting): extract reusable next-episode helpers

utils/casting/episodes.test.ts

@@ -0,0 +1,19 @@
+import { describe, expect, test } from "bun:test";
+import { findNextEpisode } from "./episodes";
+
+const ep = (id: string) => ({ Id: id });
+
+describe("findNextEpisode", () => {
+  test("returns the episode after the current one", () => {
+    expect(findNextEpisode([ep("a"), ep("b"), ep("c")], "b")).toEqual(ep("c"));
+  });
+  test("returns null on the last episode", () => {
+    expect(findNextEpisode([ep("a"), ep("b")], "b")).toBeNull();
+  });
+  test("returns null when the current id is not found", () => {
+    expect(findNextEpisode([ep("a"), ep("b")], "x")).toBeNull();
+  });
+  test("returns null for an empty list", () => {
+    expect(findNextEpisode([], "a")).toBeNull();
+  });
+});