docs: add S7 PLC simulator implementation plan

- Chunk 1: Project infrastructure (solution, projects, packages)
- Chunk 2: Core enums and entities (PLC type, instance config/state)
- Chunk 3: Memory storage implementation (M/DB/I/Q/T/C regions)
- TDD approach with comprehensive unit tests
- Bite-sized tasks with exact file paths and commands

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>