Discriminate prefetch() from URL data in static shell validation
With `ensureStatic` at `'shell'` or higher, validation renders a static App
Shell, where `prefetch()` and URL data both resolved in `PrefetchStatic` and
an unguarded `prefetch()` was reported as URL data.
Add `PrefetchStatic_prefetch`, a discrimination-only stage between
`ShellStatic` and `PrefetchStatic`. `prefetch()` resolves there in static
renders, and the static validation sequence maps it to the prefetch() hole
kind, mirroring `PrefetchRuntime` on the runtime side. Nothing else is gated
on the stage, and `advanceStage` resolves it on the way to `PrefetchStatic`.
Update the prefetch()-outside-Suspense snapshots under `'shell'`,
`'prefetch'`, and `'navigation'` to expect the prefetch() message.
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>