Merge remote-tracking branch 'upstream/codex/implement-battle-of-the-kings-variant-7bhqbn'

[Belzedar94]

Summary

• add the Battle of the Kings variant with staged gating progressions, extinction rules, and variant registration
• extend gating logic so forced gating sequences and gating-square safety checks are enforced when generating and playing moves
• expand the Python tests to cover Battle of the Kings start FENs, gating sequences, en passant interactions, and extinction behaviour

Testing

• make -j2 ARCH=x86-64 build
• ./stockfish check variants.ini

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https://chatgpt.com/codex/tasks/task_e_68d4512aa6e4832298cf90d3447d9d20