Add a Turbopack error for missing root layouts (#97639)
## Summary
Turbopack currently builds an App Router page without a root layout,
leaving the problem to surface later at runtime. Report it while
building the app structure instead:
`route/page.js doesn't have a root layout. To fix this error, make sure
every page has a root layout.`
The check uses the layouts already inherited during directory-tree
traversal, so route groups and apps with multiple root layouts keep
working. Route handlers are unaffected.
Webpack behavior is intentionally unchanged in this PR.
## Verification
- `HEADLESS=true pnpm test-dev-turbo
test/e2e/app-dir/create-root-layout/create-root-layout.test.ts`
- `HEADLESS=true pnpm test-start-turbo
test/e2e/app-dir/create-root-layout/create-root-layout.test.ts`
- `HEADLESS=true pnpm test-dev-webpack
test/e2e/app-dir/create-root-layout/create-root-layout.test.ts`
- `HEADLESS=true pnpm test-start-webpack
test/e2e/app-dir/create-root-layout/create-root-layout.test.ts`
- `HEADLESS=true pnpm test-dev-turbo
test/e2e/app-dir/root-layout/root-layout.test.ts`
