Fix SSR runtime failures for React Server Components

[justin808]

Summary

Fixes the 37/38 rspec test failures on the RSC branch caused by SSR runtime errors after switching from ExecJS to the react-on-rails-pro NodeRenderer.

• CI: start Node renderer service — The RSC branch requires the NodeRenderer at port 3800 for SSR, but CI never started it, causing Net::ReadTimeout on all prerendered pages. Added a startup step with TCP readiness check and RENDERER_PASSWORD env var.
• Server bundle: use fallbacks instead of externals — The previous commit externalized Node builtins (path/fs/stream) as CommonJS require() calls, but the Node renderer's vm.createContext() sandbox has no require. Reverted to resolve.fallback: false which stubs these unused code paths at build time.
• Server bundle: polyfill MessageChannel — react-dom/server.browser.js instantiates MessageChannel at module load. The Node renderer VM lacks this global (unlike Bun used by ExecJS on master). Added a BannerPlugin polyfill.
• RouterApp.server.jsx: add 'use client' directive — The auto-bundling system misclassified this traditional SSR component (uses StaticRouter) as a React Server Component because it lacked 'use client'. Added the directive so it registers via ReactOnRails.register() instead of registerServerComponent().

All 38 rspec tests pass locally.

Test plan

• bundle exec rspec — all 38 tests pass (0 failures)
• bundle exec rubocop — no offenses
• All 3 bundles (client, server, RSC) build successfully
• CI passes on this branch

🤖 Generated with Claude Code

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Note

Medium Risk
Touches the SSR/RSC rendering pipeline (NodeRenderer, webpack configs, routing) and upgrades core dependencies, which could break rendering or CI if configuration/env vars are wrong.

Overview
Migrates the app to React on Rails Pro + NodeRenderer for SSR and React Server Components, including a new initializer (react_on_rails_pro.rb) that enables RSC, configures renderer URL/auth, and adds the rsc_payload_route plus a /server-components page.

Adds a third Shakapacker/Rspack build target (rscWebpackConfig.js) and a custom RspackRscPlugin to emit React Flight manifests, updates webpackConfig.js to build client/server/RSC (or select via *_BUNDLE_ONLY), and adjusts SSR bundling to work in the NodeRenderer VM sandbox (CommonJS output, Node-builtin fallbacks, and a MessageChannel polyfill banner).

Updates CI/dev workflow to start and health-check the Node renderer (including RENDERER_PASSWORD), introduces a renderer launcher script + cache ignore, and updates client code to consistently import react-on-rails-pro with added 'use client' directives where needed; includes docs/QA updates and dependency bumps (Rails patch, React/Shakapacker/React on Rails Pro).

^{Reviewed by Cursor Bugbot for commit 9a71217. Bugbot is set up for automated code reviews on this repo. Configure here.}

Summary by CodeRabbit

• New Features
  • RSC Demo page added (accessible from navigation) showing server environment, streamed comments with animated skeleton, and interactive toggle panels for selective hydration.
• Updates
  • Improved server-rendering and React Server Components support for streaming, selective client hydration, and more responsive interactive experiences.