Refine Phase 3 Plan and remove outdated PLAN.md

- Updated the Phase 3 Plan to clarify the remaining work on ZeroProxy compatibility, route containment, navigation integrity, stealth membrane, and WebSocketStream.
- Deleted the outdated PLAN.md file, which contained legacy design and requirements that are no longer relevant.
- Revised the README.md to reflect the current status of the project, emphasizing the prototype phase and linking to the updated Phase 3 Plan.
- Enhanced clarity in the README regarding the implementation details and requirements.

Author: lemon-mint