New game mode: Royals mode

In Royals mode, only kings and queens will spawn. However, kings cannot move to a square where they would be taken on the next move.

- Implement this mode
- Test if it is perhaps too hard...

No files, tests, or entry points are referenced. Start by finding where existing game modes are defined and where piece spawning and king-move legality are validated, then model Royals mode behavior on similar rules. Run the project’s tests (or any mode/rule tests if present) after edits. Done when Royals mode spawns only kings and queens and blocks king moves that allow immediate capture.

Scritto dal modello di indicizzazione a partire dal testo della issue.