Introduces GameContext (board + per-side CastlingRights), CastleSide enum,
and a Board.withCastle extension method. Also pins missing verification
checksums for com.fasterxml:oss-parent:41 and org.junit:junit-bom:5.9.2.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>