Propagate subtree prefetch hints to the route tree root
The prefetch scheduler decides whether a `<Link prefetch={true}>` does
a full prefetch (dynamic data included) or a partial, static-only one
by reading a single hint at the root of the route tree: the bit that
records whether any segment beneath it opted into Partial Prefetching.
For that to be correct, an opt-in on a deeply nested segment — a page
with `unstable_prefetch = 'partial'`, say — has to propagate all the
way up to the root.
Two places build a route tree and so must perform this propagation:
the server, when it generates the prefetch route tree, and the client,
when it merges a navigation patch into the existing tree. The client
merge was copying each cloned segment's hints verbatim from the
previous tree instead of recomputing them from the merged children, so
the two could disagree. Pull the propagation into a shared helper used
by both.
A regression test covers a route whose opt-in lives several layout
levels below the root and asserts that a `prefetch={true}` to it
prefetches only the static shell, exercising propagation through the
intermediate layouts.
