[KnightStackSys]

Summary

• Add variant: spell_chess alongside existing default four_player_teams mode.
• Pair candidate and baseline by White/Black instead of 4PC teams, supporting SPRT and SPSA without modifying their statistics.
• Support complete freeze@d5:e2e4 / jump@d4:d1d8 spell-turn notation, standard/extended SpellWarden FEN openings, and legal opening validation.
• Use SpellWarden's native UCI diagnostics to distinguish checkmate/stalemate and adjudicate terminal king captures from the 8×8 board FEN, not 4PC mate-1 heuristics.
• Add SpellWarden config example, docs, and regression tests.

Dependency

Requires the companion SpellWarden change that makes d emit info string check: yes/no. This PR keeps all existing IronPhoenix configurations working with the unchanged default variant.

Validation

GitHub CI pending; no live engine-versus-engine benchmark claimed.