Add comprehensive implementation plan for JS Pro package separation

This plan documents all architectural decisions and step-by-step
implementation for PR #4: splitting JavaScript Pro functionality
into a separate react-on-rails-pro package.

Key decisions documented:
- Pro package uses core as dependency (not peer dependency)
- Caret range versioning strategy following React model
- Dual registry system with direct imports (MIT→MIT, Pro→Pro)
- Code reuse strategy layering Pro over Core functionality
- Feature split based on force-load commit analysis

Implementation broken into 10 major steps with 30+ checkpoints,
comprehensive testing strategy, and rollback procedures.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: AbanoubGhadban