Shim `next/root-params` module with runtime getters

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Summary

Next.js added generated type definitions for next/root-params, which provides runtime getter functions for root layout parameters. The module already existed at runtime but now has build-time type generation that produces a root-params.d.ts file under .next/types.

Upstream

• Commit: vercel/next.js@46e2114
• PR: feat: generate root params types vercel/next.js#91019

What changed

• next/root-params exports async getter functions (e.g., lang(), locale()) for each root param discovered from App Router root layouts
• Type generation: .next/types/root-params.d.ts is produced during next build, next dev, and next typegen
• Root params are collected from layout routes in the route types manifest by scanning for the shallowest layout in each branch (root layouts)
• Params can be string, string[], or undefined depending on whether they're optional, catch-all, or missing from some roots
• Feature is enabled when experimental.rootParams or cacheComponents is true
• The generated types are wired into next-env.d.ts via an ESM import

Impact on vinext

vinext needs to:

1. Add next/root-params to the shim map in resolveId so imports resolve correctly
2. Implement the runtime module — since vinext scans app/ directories for routing, it can detect root layout params and generate the appropriate getter functions
3. Consider type generation if vinext supports TypeScript project references or next-env.d.ts equivalent

This is primarily relevant for App Router apps that use root-level dynamic segments (e.g., app/[lang]/layout.tsx). The runtime behavior is the higher priority; type generation can follow later.