Suppress prefetch={true} warning when route opts out via instant = false (#95099)
In dev with Cache Components, navigating via a `<Link prefetch={true}>`
to a route that hasn't opted into Partial Prefetching logs a warning,
since the full prefetch pulls in dynamic data and defeats the
static/dynamic split.
`instant = false` is the explicit API for opting a route out of this
validation, so it should silence the warning. Previously it didn't: an
`instant = false` segment set no prefetch hint at all, so the warning
still fired.
Add a `SubtreeHasInstantFalse` PrefetchHint, set on any segment that
exports `instant = false` and propagated up to the root (mirroring
`SubtreeHasPartialPrefetching`). The dev warning now skips routes where
this bit is present anywhere on the target subtree.
This is warning-only and kept separate from the existing
`PrefetchDisabled` bit, which feeds `StaticPrefetchDisabled` and would
change actual prefetch behavior.