Support named typed-path nodes for index routes

[brenelz]

Summary

Allow an index route such as / to expose a named typed-path node, so applications can write paths.index instead of calling the root builder with paths().

Current behavior

const Router = createRouter({
  routes: [
    { path: "/", component: Home },
    { path: "/about", component: About },
  ],
});

const { paths } = Router;

paths();       // "/"
paths.about;   // "/about"
paths.index;   // TypeScript error; the runtime proxy represents "/index"

Static path nodes have descriptive property names, but the root/index route is represented by calling the path proxy itself. This makes a navigation vocabulary such as paths.index, paths.about, and paths.settings impossible without maintaining a separate application-level alias map.

Desired API

A route should be able to opt into a named path node whose value still comes from its actual path:

paths.index; // "/"

The exact route configuration API is open for discussion. Collision-safe possibilities include explicit naming:

{ path: "/", name: "index", component: Home }

or explicit index metadata:

{ path: "/", index: true, component: Home }

An explicit convention seems preferable to globally treating .index as a no-op segment, since applications may have a real /index route.

Design considerations

• Keep the runtime proxy and RoutePaths inference aligned.
• Preserve access to literal /index routes without ambiguity.
• Consider nested index routes, for example paths.users(id).index producing /users/:id.
• Preserve base-path and hash-history rendering behavior.
• Keep paths() working for backward compatibility.
• Ideally allow names other than index if the underlying feature is named routes rather than an index-only special case.

Motivation

This keeps links, redirects, and navigation calls on one typed, semantic path vocabulary without requiring manually maintained aliases:

<a href={paths.index}>Home</a>
<a href={paths.about}>About</a>