Add an AI-verifiable maintenance environment

[infeng]

Summary

• document the supported environment, code map, verification contract, and observable fixtures in AGENTS.md
• add a single npm run verify command covering tests, lint, library build, demo build, SSR build, and package contents
• extend CI so Node.js 22 and 24 also validate the browser demo and SSR fixture
• restore the Webpack 4 demo by using its compatible HtmlWebpackPlugin major
• restore the SSR fixture on modern Node.js and model the supported client-only Viewer import

Why

Future maintenance will be performed with coding agents. Agents need deterministic setup, one-command acceptance, clear failure logs, and real browser/SSR environments that can be inspected before changes are merged.

Validation

• npm ci --ignore-scripts
• npm run verify
• browser demo compiled and served at http://127.0.0.1:8001/
• SSR fixture returned server-rendered HTML and a 200 response for /main.js at http://127.0.0.1:8005/
• all package-lock URLs use registry.npmjs.org