I got fastify to work! There was an interesting challenge point with an error looking like this:

```
Server Fn Error!

TypeError: Response body object should not be disturbed or locked
    at extractBody (node:internal/deps/undici/undici:5574:17)
    at new Request (node:internal/deps/undici/undici:9809:48)
```

But I was able to solve that by adding a bunch of content type parser no-op functions, inspired by this Next.js issue: https://github.com/vercel/next.js/issues/24894

Here's my code:

```
import { join } from 'node:path';
import fastifyStatic from '@fastify/static';
import Fastify from 'fastify';
import { toNodeHandler } from 'srvx/node';
import type { FastifyContentTypeParser } from 'fastify';

/**
 * This Fastify server is used to serve the built app from the dist directory for deployed
 * environments. For local development, this server is not used -- `vite dev` is used instead.
 */
const fastify = Fastify({
	logger: true,
	routerOptions: { ignoreTrailingSlash: true },
	trustProxy: true,
});

fastify.register(healthzEndpoints);

// Production only - serve the built app
const { default: handler } = await import('./dist/server/server.js');
const nodeHandler = toNodeHandler(handler.fetch);

// CRITICAL: No-op content parsers to prevent Fastify from consuming the body
// This allows srvx/TanStack Start to read the body stream
const noOpParser: FastifyContentTypeParser = (_req, payload, done) => {
	return done(null, payload);
};

// Register no-op parsers for all content types that might have a body
fastify.addContentTypeParser('application/json', noOpParser);
fastify.addContentTypeParser('application/x-www-form-urlencoded', noOpParser);
fastify.addContentTypeParser('multipart/form-data', noOpParser);
fastify.addContentTypeParser('text/plain', noOpParser);
// Catch-all for any other content types
fastify.addContentTypeParser('*', noOpParser);

await fastify.register(fastifyStatic, {
	root: join(import.meta.dirname, 'dist/client/assets'),
	prefix: '/temp/my-path/assets',
});

// Handle requests through TanStack Start
fastify.all('/temp/my-path*', async (request, reply) => {
	reply.hijack();
	await nodeHandler(request.raw, reply.raw);
});

try {
	await fastify.listen({
		host: process.env.SERVER_HOST,
		port: process.env.SERVER_PORT,
	});
} catch (err) {
	fastify.log.error(err);
	process.exit(1);
}
```