Refactor: Navigate and refresh simultaneously (#86747)
This is a refactor of the ppr-navigations module to support navigating
to a new tree and refreshing shared layouts within the same navigation.
Rather than modeling navigations and refreshes/revalidations as separate
operations, a refresh is considered a special case of a navigation where
any existing dynamic data is disregarded, and new data is fetched from
the server. The rest of the tree is diffed as usual, and the routing
behavior is the same.
Previously, the ppr-navigations module would revert to the "create" path
whenever it was missing dynamic data (the CacheNode) for a shared
segment. Now, the FlightRouterState alone is used to diff the old and
new trees.
When dynamic data is missing, any shared segment (one that exists in
both the old and new trees) is considered to be part of a "refresh", and
any segment that is present only in the new tree is considered to be
part of a "navigation". There are subtle distinctions in behavior
between refreshes and navigations. For example, refreshed page segments
aren't scrolled to, but navigated page segments are.
