feat: generate root params types (#91019)
## Summary
Generate type definitions for `next/root-params` from App Router root
layouts.
This adds a `root-params.d.ts` file under `.next/types` during build,
`next dev`, and `next typegen`, so imports from `next/root-params` get
accurate autocomplete and type checking.
## What Changed
- Collect root param metadata from layout routes in the route types
manifest
- Generate `.next/types/root-params.d.ts` for `next/root-params`
- Emit root param types in `next build/dev/typegen`
- Infer `undefined` only when a param is not present in every root
layout
- Model mixed root param shapes correctly as unions
- For example, `string | string[] | undefined` when the same param name
appears with different shapes across roots
- Remove stale `root-params.d.ts` when the feature is disabled or there
are no root params
## Why
`next/root-params` already works at runtime, but its types were not
tailored to the current app.
This change closes that gap by generating declarations from the app's
actual root layouts. Required params stay required for single-root apps,
while multi-root apps correctly expose params that may be missing or
have mixed segment shapes.
## Testing
- Added focused unit tests for root param type generation
- Added a typecheck e2e test for generated `root-params.d.ts`
