Remove clib dependency, use pure Python implementation only

This commit completes the migration to a pure Python S7 protocol
implementation, removing the dependency on the native Snap7 C library.

Changes:
- Remove snap7/clib/ folder (ctypes bindings)
- Remove snap7/native/ folder (move contents to snap7/)
- Remove snap7/common.py, snap7/protocol.py, snap7/protocol.pyi
- Flatten structure: client.py, server.py, partner.py at top level
- Add connection.py, datatypes.py, s7protocol.py for protocol handling
- Simplify CI/CD workflows (no native library builds needed)
- Update README.rst and CLAUDE.md for pure Python architecture
- Update pyproject.toml (remove native lib package-data)
- Update all tests to work with native implementation

The package is now a pure Python wheel that works on all platforms
without architecture-specific builds.

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: gijzelaerr