Refactor content lifecycle modules

[heznpc]

Summary

• split content lifecycle, message routing, DOM observer, and term preview into focused content helpers
• keep the existing _sb contract while reducing content.js responsibilities
• make stream-cancel E2E deterministic and give batched E2E runs more timeout headroom

Tests

• npm run format:check
• npm run lint
• npm run typecheck
• npm test
• npm run build:bundle
• npm run build:firefox
• npm run test:e2e