Enhance Tetris simulation and RNG handling in tetris6x6 module

- Updated the random number generation mechanism to use an 8-bit LFSR for improved synthesis compatibility and to prevent constant-folding issues.
- Refactored game logic to ensure proper piece spawning and movement, enhancing gameplay experience.
- Improved game-over detection and row validation to ensure accurate game state representation.
- Cleaned up comments and code structure for better readability and maintainability.

Author: MacroModel