feat(email-renderer): generate bounded animated countdown images

packages/email-renderer/test/countdown.test.ts

@@ -0,0 +1,22 @@
+import { expect, test } from "bun:test";
+import { countdownLabel, renderCountdownGif } from "../src/countdown.ts";
+
+test("countdown ticks across boundaries and stops at zero", () => {
+  expect(countdownLabel(86401000, 0)).toBe("001D 00H 00M 01S");
+  expect(countdownLabel(86401000, 2000)).toBe("000D 23H 59M 59S");
+  expect(countdownLabel(1000, 2000)).toBe("000D 00H 00M 00S");
+  expect(() => countdownLabel(NaN, 0)).toThrow();
+  expect(() => renderCountdownGif(1000, 0, 61)).toThrow();
+});
+
+test("countdown generates deterministic bounded non-looping GIF output", () => {
+  const first = renderCountdownGif(60000, 0);
+  expect(first).toEqual(renderCountdownGif(60000, 0));
+  expect(new TextDecoder().decode(first.subarray(0,6))).toBe("GIF89a");
+  expect(first[6]! + first[7]! * 256).toBe(320);
+  expect(first[8]! + first[9]! * 256).toBe(64);
+  expect(first.at(-1)).toBe(0x3b);
+  expect(first.byteLength).toBeLessThan(100_000);
+  expect(new TextDecoder().decode(first)).not.toContain("NETSCAPE2.0");
+  expect(renderCountdownGif(0, 1000).byteLength).toBeLessThan(first.byteLength);
+});