App Router: misc correctness stragglers (root-layout redirect, global-not-found default, esm `.js`, react@experimental, middleware-base-path)

> *This issue was created by an agent auditing CI failures from the [Next.js Deploy Suite](https://github.com/cloudflare/vinext/actions/runs/27113612207) (vinext `main` vs Next.js `v16.2.6`, 2026-06-08).*

## Problem

A handful of small, independent App Router correctness failures with no shared root cause and no existing tracking issue. Grouped here for triage; split out if any turns out to be substantial.

## Affected Test Suites

- `test/e2e/app-dir/root-layout-redirect/root-layout-redirect.test.ts` (1 — "should work using browser"; redirect issued from the root layout)
- `test/e2e/app-dir/global-not-found/not-present/not-present.test.ts` (1 — "should render default 404 when global-not-found is not defined but enabled")
- `test/e2e/app-dir/app-esm-js/index.test.ts` (1 — "should be able to render nextjs api in app router"; `.js` ESM extension)
- `test/e2e/app-dir/rsc-basic/rsc-basic-react-experimental.test.ts` (1 — "should opt into the react@experimental when enabling $flag")
- `test/e2e/middleware-base-path/test/index.test.ts` (1 — "should execute from absolute paths")

## Estimated impact

~5 e2e failures across 5 suites.

## Recommendation

Reproduce and triage each independently; promote any non-trivial one to its own issue. `middleware-base-path` is adjacent to the closed basePath issues (#1329 / #1333).