Add Start.export for serverless environments

[nounder]

Start.serve owns the full server lifecycle by calling Bun.serve() and BunRuntime.runMain. Serverless platforms like Cloudflare Workers or Deno Deploy expect a fetch handler export.

Add Start.export that accepts layers same as Start.pack but returns a fetch(request: Request): Promise<Response> instead of starting a server.

[nounder]

The behavior of fetch should be configurable by layers to enable context pulling for Cloudflare (fetch(req, ctx))