Add comprehensive test coverage for pin_lock and SiliconPlatform

- Add tests for pin_lock.lock_pins with mocked dependencies (96% coverage)
- Add tests for SiliconPlatformPort.__repr__ method
- Add tests for CF20 package definition
- Add tests for PinSignature.annotations method
- Add tests for IOBuffer and FFBuffer with invalid port types
- Document bug in _BasePackageDef.sortpins method
- Increase overall project test coverage to 95%

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

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

Author: robtaylor