fix(email-renderer): stack columns only at compact widths

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

@@ -2,6 +2,7 @@ import { describe, expect, test } from "bun:test";
 import { parse, type DefaultTreeAdapterMap } from "parse5";
 import {
   emailCoreBlocks,
+  emailKind,
   renderEmail,
   validateAmpEmail,
   validateMjml,
@@ -56,6 +57,14 @@ const document: BlockDocument = {
 };
 
 describe("email renderer", () => {
+  test("responsive stacking begins below the desktop email width", async () => {
+    const rendered = await renderEmail(document);
+    expect(rendered.ok).toBe(true);
+    if (!rendered.ok) return;
+    expect(emailKind.breakpoints.compact).toBe(480);
+    expect(rendered.value.parts.html.content).toContain("max-width:480px");
+    expect(rendered.value.parts.html.content).not.toContain("max-width:620px");
+  });
   test("countdown URLs are derived for HTML AMP and MJML without mutating the document", async () => {
     const draft = structuredClone(document);
     draft.root.slots.content[0]!.slots.content = [{id:"timer",type:"content.countdown",version:1,props:{label:"Offer ends",deadline:"2026-10-01T00:00:00Z",fallback:"Offer ended"},styles:{},slots:{}}];