Add king's diagonal face-off support (closes issue #188)

[Belzedar94]

• Introduced a configurable diagonalGeneral rule and propagated it through variant parsing, castling safety, move legality, SEE, and chase detection so diagonal king face-offs are blocked whenever a variant opts into the restriction.
• Implemented Eurasian Chess, wiring in its 10×10 start position, cannon and vao pieces, promotion limits, king mobility zones, and enabling both rook and diagonal face-off rules before registering the variant with the large-board lineup.
• Documented the new diagonal face-off toggle in variants.ini and extended the Python suite to expose the Eurasian start FEN, list the variant, and verify diagonal visibility prevents illegal king encounters.

This also contains PRs: #919 and #912

This was vibecoded so a review would be needed!