feat: MenuNav layer for the SummerCMS menu API

- typed client and useMenu/useMenuApi (SSR, menu:<code>:<locale> key,
  server and public bases, x-forwarded-for forwarded also on cached renders)
- MenuNav and recursive MenuNavItem: WAI disclosure navigation, URL
  allowlist with a text fallback, rel on _blank, active link and trail,
  mobile toggle, Esc / focus-out / outside pointer / route change closing
- menu tokens and base CSS, README
- node tests (link rules, client, server-rendered NavItem) and a playground
  SSR smoke against a fixture API

tests/render.ts

@@ -0,0 +1,60 @@
+// Test helper: compiles app/components/Menu/NavItem.vue with Vue's SFC
+// compiler and renders it with the server renderer. NuxtLink is stubbed as
+// a plain anchor (it renders one for a path in the app too), the component
+// resolves itself for the recursion, and the returned HTML has no hydration
+// comments.
+import { readFileSync, rmSync, writeFileSync } from 'node:fs'
+import { dirname, join } from 'node:path'
+import { fileURLToPath, pathToFileURL } from 'node:url'
+import { compileScript, parse } from 'vue/compiler-sfc'
+import { createSSRApp, defineComponent, h, type Component } from 'vue'
+import { renderToString } from 'vue/server-renderer'
+import type { MenuEntry } from '../app/utils/types.ts'
+
+const here = dirname(fileURLToPath(import.meta.url))
+const source = join(here, '..', 'app', 'components', 'Menu', 'NavItem.vue')
+
+let compiled: Promise<Component> | null = null
+
+async function compileNavItem(): Promise<Component> {
+  const { descriptor } = parse(readFileSync(source, 'utf8'), { filename: source })
+  const script = compileScript(descriptor, { id: 'navitem', inlineTemplate: true, genDefaultAs: '__sfc__' })
+  // The compiled module keeps the relative imports, so it is written next to
+  // the source, imported (Node strips the types) and removed.
+  const out = join(dirname(source), `.navitem-${process.pid}.compiled.ts`)
+  writeFileSync(out, `${script.content}\nexport default __sfc__\n`)
+  try {
+    const mod = (await import(pathToFileURL(out).href)) as { default: Component }
+    return mod.default
+  } finally {
+    rmSync(out, { force: true })
+  }
+}
+
+const NuxtLink = defineComponent({
+  props: { to: { type: String, required: true } },
+  setup(props, { slots, attrs }) {
+    return () => h('a', { href: props.to, ...attrs }, slots.default?.())
+  },
+})
+
+export async function renderNavItem(opts: { entry: MenuEntry; routePath: string; open?: boolean }): Promise<string> {
+  compiled ??= compileNavItem()
+  const NavItem = await compiled
+  const app = createSSRApp({
+    render: () =>
+      h(NavItem, {
+        entry: opts.entry,
+        depth: 0,
+        idBase: 'm-0',
+        open: opts.open ?? false,
+        routePath: opts.routePath,
+        host: 'site.test',
+        submenuLabel: 'Show submenu: :title',
+      }),
+  })
+  app.component('NuxtLink', NuxtLink)
+  app.component('MenuNavItem', NavItem)
+  // Hydration markers (<!--[-->, <!--v-if-->) are dropped for readable assertions.
+  return (await renderToString(app)).replace(/<!--[^>]*-->/g, '')
+}