Add framework-free nitro preview repro for the aws-lambda 404

Standalone nitro + Vite build with a single route. Reproduces the preview
404 stub with preset: 'aws-lambda' (srvx loadServerEntry has no branch for
the handler-only export); returns 200 when swapped to preset: 'node-server'.

Targeted by nitrojs/nitro#4052.

Author: martinmiglio

nitro-only/README.md

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+# nitro-only preview 404 repro
+
+Framework-free reproduction of the nitro preview bug that surfaces in TanStack Start + prerender.
+Nitro + Vite, no framework plugins, no other wrappers.
+
+## Bug
+
+With `preset: "aws-lambda"`, `nitro preview` returns **HTTP 404** on every route. With `preset: "node-server"` (swap the line in `nitro.config.ts`), the same routes return **200 OK**.
+
+Root cause: aws-lambda's runtime exports only `{ handler }`. srvx's `loadServerEntry` fetch-extraction cascade (`src/loader.ts:163-178`) has no branch that matches `{ handler }`, so `entry.fetch` returns `undefined`. `nitro/src/preview.ts:74-75` keeps its default `() => new Response("Not Found", { status: 404 })` stub, and serves it for every request.
+
+## Reproduce
+
+```bash
+cd nitro-only
+npm install
+npx nitro build
+npx nitro preview &
+sleep 1
+curl -i http://localhost:3000/hello
+# HTTP/1.1 404
+# content-type: text/plain;charset=UTF-8
+# Not Found
+
+kill %1
+```
+
+Change `preset: "aws-lambda"` to `preset: "node-server"` in `nitro.config.ts`, rebuild, and the same curl returns `200 OK` with body `hello`.
+
+## Versions
+
+Pinned in `package.json` at the nitro beta where the bug was first investigated; also reproduces on `nitro@nightly` at audit time.
+
+## Fix in flight
+
+[nitrojs/nitro#4052](https://github.com/nitrojs/nitro/pull/4052) adds `export default { fetch: nitroApp.fetch }` to the aws-lambda runtime, which triggers srvx's `mod.default.fetch` branch.

nitro-only/nitro.config.ts

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+import { defineConfig } from "nitro";
+
+export default defineConfig({
+  preset: "aws-lambda",
+  serverDir: ".",
+});

nitro-only/package.json

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+{
+  "name": "nitro-only-repro",
+  "private": true,
+  "type": "module",
+  "scripts": {
+    "build": "nitro build",
+    "preview": "nitro preview"
+  },
+  "dependencies": {
+    "nitro": "3.0.260311-beta"
+  }
+}

nitro-only/routes/hello.ts

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+import { defineHandler } from "nitro";
+
+export default defineHandler(() => "hello");