Embed static sibling info in route tree (#88692)
This change adds infrastructure for a planned client-side optimization
where the router can infer route structure from previously prefetched
routes, avoiding redundant route tree requests.
When the client prefetches /blog/post-1 and learns the route structure
is /blog/[slug], it could potentially reuse that knowledge for
/blog/post-2 without another server roundtrip. However, if a static
route like /blog/featured exists at the same level, the client needs to
know NOT to apply the dynamic template for that URL.
staticSiblings provides this information. When a dynamic segment like
[slug] has static siblings (e.g., "featured", "archive"), the server
includes them in the prefetch response. The client can then check
whether an incoming URL matches a known static sibling, and if so,
recognize that the previously learned route structure does not apply.
