Optimistic routing: client-side route prediction (#88965)
Based on:
- https://github.com/vercel/next.js/pull/88834
- https://github.com/vercel/next.js/pull/88989
- https://github.com/vercel/next.js/pull/88998
---
Adds optimistic routing, enabling the client to predict route structure
for URLs that haven't been prefetched yet. When navigating to a URL like
/blog/post-2, if we've previously learned the pattern from /blog/post-1,
we can predict the route structure without waiting for a server
response.
The client learns route patterns from server responses and builds a trie
indexed by URL parts. When a cache miss occurs, we check if the URL
matches a known pattern. If so, we create a synthetic cache entry from
the template, allowing immediate rendering of loading boundaries.
Static siblings (like /blog/featured alongside /blog/[slug]) are tracked
to avoid incorrect predictions — we only predict dynamic routes when
we're confident no static sibling matches.
