Support `unstable_prefetch` route segment config

## Next.js Change

Next.js added a new `unstable_prefetch` route segment config option, decoupled from `unstable_instant`.

**Commit:** https://github.com/vercel/next.js/commit/ac6d9932a64aa1f927c5dda2f70a39da4dc777fd
**PR:** [#92754](https://github.com/vercel/next.js/pull/92754)

## What Changed

A new route segment config export `unstable_prefetch` was added to App Router pages/layouts:

```ts
export const unstable_prefetch = 'runtime' // or 'static'
```

Key details:
- **Type:** `'static' | 'runtime'` — `'static'` is a no-op (default), `'runtime'` enables runtime prefetching
- **Requires `cacheComponents: true`** in `next.config.js` — throws a build error otherwise
- **Server Components only** — throws a build error if used in a `"use client"` module
- **Decoupled from `unstable_instant`** — previously, runtime prefetching was configured via `unstable_instant: { prefetch: 'runtime' }`. Now `unstable_prefetch` controls prefetching independently, and `unstable_instant` is purely for instant validation
- The `create-component-tree.tsx` now reads `unstable_prefetch` instead of `unstable_instant.prefetch` to determine `hasRuntimePrefetch`

## Impact on vinext

vinext's route segment config parsing (`routing/app-router.ts`, `get-page-static-info` equivalent) should recognize `unstable_prefetch` as a valid export to avoid spurious warnings. The actual runtime prefetching behavior is not something vinext needs to implement immediately (it's experimental and tied to `cacheComponents`), but the config should be parsed and acknowledged.

## Files Changed (non-test)

- `packages/next/src/build/segment-config/app/app-segment-config.ts` — added `PrefetchSchema`, `Prefetch` type, and `unstable_prefetch` to the schema
- `packages/next/src/build/analysis/get-page-static-info.ts` — added validation (requires `cacheComponents`, disallows `"use client"`)
- `packages/next/src/server/app-render/create-component-tree.tsx` — reads `unstable_prefetch` instead of `unstable_instant.prefetch`
- `crates/next-core/src/segment_config.rs` — Turbopack-side parsing of the new config

Start in vinext's routing/app-router.ts and the get-page-static-info equivalent to trace how route segment config exports are parsed and warned about. Add recognition for unstable_prefetch without implementing runtime prefetching, then verify that valid values are acknowledged without spurious warnings.

Written by the indexing model from the issue text.