Add SPA routing with custom fetch router outputs

[brophdawg11]

@remix-run/fetch-router currently assumes every dispatch produces a Response, even when the router is used entirely in memory. This adds a configurable output contract and a remix/ui/spa runtime for building browser-only applications whose routers map URLs directly to rendered UI.

• Adds ambient RouterTypes.output configuration across routers, middleware, controllers, mounted routes, and nested context.router.fetch calls while retaining the existing string | URL | Request input contract.
• Preserves an internal Request for every dispatch, including propagation from router.fetch(url, { signal }) to context.request.signal.
• Strongly types response-mutating middleware as requiring Response output and throws at runtime if downstream middleware returns another value.
• Exports SPA and the lower-level createSPA setup utility from remix/ui/spa. They intercept same-origin Navigation API navigations, cancel superseded loads, expose active and pending URLs, and respect rmx-document and rmx-history.
• Preserves form methods and forwards non-GET FormData. By default, same-URL submissions replace history while different-URL submissions push; rmx-history can override this behavior. History traversals revisit form destinations with GET because navigation entries do not retain submitted data.
• Adds a Vite demo in demos/spa showing route maps, controllers, render middleware, pending navigation UI, cancellation, and client-side form submissions.

import { createRouter } from 'remix/router'
import { createRoot, type RemixNode } from 'remix/ui'
import { SPA } from 'remix/ui/spa'

declare module 'remix/router' {
  interface RouterTypes {
    output: RemixNode
  }
}

let router = createRouter({
  defaultHandler: () => <h1>Not Found</h1>,
})

router.get('/', () => <h1>Home</h1>)
router.get('/about', () => <h1>About</h1>)

let root = createRoot(document.getElementById('app')!)
root.render(<SPA router={router} fallback="Loading…" />)

[github-actions]

Preview Build Available

A preview build has been created for this PR. You can install it using:

pnpm install "remix-run/remix#preview/pr-11629&path:packages/remix"

This preview build will be updated automatically as you push new commits.

[brophdawg11]

actions now return Remix nodes

[brophdawg11]

Form submissions are also handled by the SPA router component

[brophdawg11]

Components can access active/pending URLs from the SPA context

[brophdawg11]

This is the important part to enable returning RemixNode's from the router