Unify RouteTree and CacheNodeSeedData on the client (#96406)
The client used to represent a server response as two parallel trees: a
RouteTree describing the route structure, and a CacheNodeSeedData tree
carrying the rendered output for each segment. The two are meant to be
isomorphic, but nothing enforced that — every consumer walked them in
lockstep and had to defend against mismatches between them.
This PR adds a `data` field to the RouteTree type and makes it generic
over a per-segment payload. For a server response, the type is
RouteTree<RSCSegmentData | null>. This removes the need to pass a
separate CacheNodeSeedData through the client navigation algorithm in
ppr-navigations.ts.
This is a step toward replacing the RSC response transport format: with
the client consuming a single unified tree, the wire format can move to
one as well.
