Split out unused chrome.debugger behavior into separate class so ModCDPServer gets simpler

[pirate]

Summary

Splits the service-worker server upstream path into explicit transport classes so ModCDPServer no longer owns loopback-vs-chrome.debugger branching after setup.

This PR includes:

• ServerUpstreamTransport base class + plus Loopback and Debugger implementations
• generic downstream transport classes for reverse websocket, native host, and NATS lifecycle
• a slimmer service worker entrypoint that uses ModCDPServer directly
• typed upstream event routing through AutoSessionRouter
• cross-language demo and routed-default test updates that stay focused on transport behavior

Validation

• pnpm exec tsc -p tsconfig.json --noEmit
• pnpm run build
• pnpm exec vitest run js/test/test.ServerUpstreamTransport.ts js/test/test.ReverseWebSocketUpstreamTransport.ts js/test/test.NatsUpstreamTransport.ts --fileParallelism=false --maxWorkers=1
• pnpm exec prek run --all-files