App Router ignores `reactStrictMode` in `next.config` (Pages Router only today)

## Summary

`reactStrictMode` appears to be applied only for the Pages Router in vinext, but not for the App Router.

Based on current code/comments, this seems known internally (`reactStrictMode` is marked as partial), but I couldn't find a dedicated issue tracking App Router parity.

## Current behavior

- `reactStrictMode: true` enables StrictMode behavior for Pages Router client rendering.
- App Router is not wrapped/enforced the same way, so StrictMode checks are not activated there.

## Expected behavior

For Next.js parity, App Router should respect strict mode behavior as well (or document a clear intentional divergence).

## Evidence

In `packages/vinext/src/check.ts`, `reactStrictMode` is explicitly marked as partial with detail similar to:

> enforced for the Pages Router ... App Router is not yet wrapped (Next.js defaults App Router strict mode on)

## Reproduction (high level)

1. Create/use an App Router app on vinext.
2. Set `reactStrictMode: true` in `next.config`.
3. Add a small component that makes StrictMode dev behavior obvious (e.g. effect/log patterns).
4. Compare with equivalent Pages Router setup and/or upstream Next.js behavior.

## Questions

1. Is this a known parity gap to be fixed?
2. Should we track this as a dedicated compatibility issue?
3. If intentional for now, can we document it more prominently in compatibility docs/check output?

Start with the partial `reactStrictMode` entry in `packages/vinext/src/check.ts`, then reproduce the App Router behavior described in the issue and compare it with the Pages Router setup. Done means App Router strict-mode behavior matches the expected Next.js behavior, or the intentional divergence is clearly documented in the compatibility output or documentation.

Written by the indexing model from the issue text.