Remove inert experimental.viewTransition flag (#96098)
## What
Removes `experimental.viewTransition` — the flag is inert. Code only;
all docs changes ship in #96097 so the two can be reviewed
independently.
## Why
Nothing in the runtime reads it: it appears only in config validation
(`config-schema.ts`) and its type/default (`config-shared.ts`).
`ViewTransition` ships in the bundled React canary, and every `<Link>`
navigation already runs inside `React.startTransition`, so view
transitions work with no configuration.
The flag was reserved for Next.js automatically adding transition types
to navigations, which never landed (confirmed with the team — a new flag
will gate that work when it's done properly). Historically it also
switched the bundled React to the experimental channel, but that stopped
being necessary when React released `ViewTransition` to canary; the
experimental channel is now selected by other flags
(`gestureTransition`, `blockingSSR`, `taint`, `transitionIndicator`).
## Changes
- `config-schema.ts` / `config-shared.ts`: schema entry, type + JSDoc,
and default removed
- `test/e2e/app-dir/view-transitions` fixture: `next.config.js` deleted
(it contained only the flag; the test never references it)
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>
