Remove `std::optional`, use enum values to represent optional piece, color, etc...

[Hardcode3]

🎯 Goal

Remove std::optional feature from the codebase in legal move generation (hot path) to optimize a bit the engine.

🧠 Key Idea / Logic

std::optional is a recent c++ feature, allowing to make a variable optional.

However, its presence in hot paths slows the engine down.

🛠️ Tasks

• Benchmark the engine before the update
• Create new enum vales representing defaults (invalid state): Piece::NO_PIECE, Color::NO_COLOR
• Remove occurences of std::optional
• Benchmark the engine after the update

✅ Acceptance Criteria

• Full test suite is passing
• Engine must be slightly faster

📝 Notes

No response