feat: Next.js Pages Router support

[andreas-karlsson]

Summary

Adds three subexports under ./pages-router/* for the Next.js Pages Router, parallel to the existing react-server/react-client pair for App Router:

• pages-router/server — withConfidence(config, gssp?) HOC that resolves the flag bundle in getServerSideProps (no exposure) and merges it into pageProps. Plus a low-level resolveConfidence helper.
• pages-router/client — <ConfidencePagesProvider> for _app.tsx. The existing useFlag / useFlagDetails from react-client work unchanged.
• pages-router/api — applyHandler({ providerName? }?) factory for the /api/confidence/apply POST route the client uses to log exposure.

The resolve token is sealed with AES-256-GCM (key from CONFIDENCE_TOKEN_KEY env var) before reaching the client, since Pages Router has no encrypted-closure equivalent to App Router server actions. The apply route opens it server-side.

README-REACT.md gets a new "Next.js Pages Router" section. The App Router quick-start was also moved to instrumentation.ts since that's router-agnostic and avoids the build-time side-effect issue with top-level-await setup files.

Test plan

• yarn typecheck clean
• yarn test — 7 token tests + 6 apply-handler tests pass
• yarn build emits dist/pages-router/{server,client,api}.{js,d.ts}
• Existing react-server / react-client builds unchanged
• Verified end-to-end against a Pages Router demo app (server-side resolve → client hook reads → apply route fires exposure)

🤖 Generated with Claude Code