Add Next.js runtime template

[lingdie]

Summary

• add Next.js v16 base and runtime image definitions
• add a TypeScript App Router project template with localized README files
• register Next.js in runtime conformance and add a legacy smoke test

Validation

• npm install / npm run build for the Next.js template
• npm audit reports 0 vulnerabilities for the template dependency tree
• bash -n for new shell scripts and runtime conformance script
• runtime-build.py plan-build resolves Debian 12.6 -> Node.js 20 -> Next.js base -> runtime
• runtime-conformance.py plan includes frameworks/next.js/v16 for en_US and zh_CN