每张卡片是一个独立演示页：真 <cmx-megasheet> 元素铺真数据，侧栏「自检」在真浏览器内跑与 verify-mN 同源的断言——绿灯亮起，代表这个里程碑此刻正真真切切活在页面上。Each card opens a standalone demo: a real <cmx-megasheet> element with real data, whose side-panel self-check runs the same assertions as verify-mN — in a real browser. Green means the milestone is genuinely alive on the page.