Add `experimental.cachedNavigations` feature flag (#90928)
The recent Cached Navigations feature introduced navigation-level
caching into the segment cache, where static and dynamic RSC data from
navigations and initial HTML loads is cached so that repeat visits can
be served instantly. This feature is currently gated behind
`cacheComponents`, meaning it activates for all projects that enable
Cache Components.
This change introduces a new `experimental.cachedNavigations` flag that
independently controls the Cached Navigations behavior. The flag
defaults to `false` so we can investigate potential regressions that the
feature might have introduced without affecting the broader Cache
Components functionality.
The flag requires `cacheComponents` to also be enabled and throws a
configuration error if `cachedNavigations` is set without it. On the
server, the staged rendering paths in `app-render.tsx` (both RSC request
handling and initial HTML rendering) now check `cacheComponents &&
cachedNavigations` before producing staged responses. On the client,
`process.env.__NEXT_EXPERIMENTAL_CACHED_NAVIGATIONS` is used alongside
`process.env.__NEXT_CACHE_COMPONENTS` for compile-time dead code
elimination in the stream tee (`app-index.tsx`) and the response
processing (`fetch-server-response.ts`).
The cached navigations test suite enables the flag via `next.config.ts`,
and the Cache Components CI jobs export
`__NEXT_EXPERIMENTAL_CACHED_NAVIGATIONS=true` to maintain test coverage.
