Cached Navigations: Cache fully static initial HTML RSC payload
When a fully static page is loaded, the RSC payload inlined in the HTML
is now written into the client-side segment cache during hydration. This
allows subsequent client-side navigations to the same route to be served
entirely from cache without any server requests, until the stale time
expires.
Server: Include a `StaleTimeIterable` (`s` field) in the
`InitialRSCPayload` during the Cache Components prerender path. The
iterable is tracked on `finalServerPrerenderStore` and closed alongside
the vary params accumulator in the sequential task after prerender
completes.
Client: Thread the stale time and head vary params from the decoded
payload through `createInitialRouterState` into a new
`writeInitialSeedDataIntoCache` function that writes each segment and
the head into the segment cache using the existing
`writeSeedDataIntoCache` / `fulfillEntrySpawnedByRuntimePrefetch`
functions.
Partially static pages (resumed at runtime) are not handled yet. Those
need byte-level truncation of the Flight stream to extract only the
static stage (same as for dynamic RSC requests).
