test(ci): add exporter heading levels and a11y typography smoke; update MAIN_PLAN

lib/exporter/zip.ts

@@ -0,0 +1,24 @@
+import JSZip from 'jszip'
+
+export type BuildZipInput = {
+  html: string
+  css: string
+  js: string
+  assets?: Record<string, Uint8Array>
+}
+
+export async function buildZip(input: BuildZipInput): Promise<Uint8Array> {
+  const zip = new JSZip()
+  zip.file('index.html', input.html)
+  zip.file('styles.css', input.css)
+  zip.file('app.js', input.js)
+  if (input.assets) {
+    for (const [path, bytes] of Object.entries(input.assets)) {
+      // In Node/Vitest, use Buffer for robust binary handling
+      const buf = Buffer.from(bytes)
+      zip.file(path, buf as unknown as Buffer)
+    }
+  }
+  const content = await zip.generateAsync({ type: 'uint8array' })
+  return content
+}