Added the Benedict Morph variant

[Natfay]

Implements the "Benedict Morph" chess variant; a morph-on-capture rule inspired by the original Benedict game, but simplified for clarity and engine compatibility.

Rules summary

• When a piece captures an opponent's piece, it morphs into the type of the captured piece (keeping its original color).
  • Example: a white bishop capturing a black knight becomes a white knight on the destination square.
• The captured piece is removed as usual.
• All other rules of orthodox chess apply (castling, en passant, promotions, check/mate conditions, etc.).
• The king is rex-exclusive:
  • The king can capture, but never morphs, it always remains a king.

Implementation details

• Added new variant factory benedictmorph_variant() in variant.cpp.
• Added four fields in StateInfo to record morph data: didMorph, morphedFrom, morphedTo, morphSquare.
• Modified Position::do_move():
  • Detect capture and perform morph before gating.
  • Skip morph if the mover is a king (rex-exclusive).
  • Update NNUE dirtyPiece list to reflect the final piece identity.
• Modified Position::undo_move() to reverse the morph.

Rex-exclusive by default. Future extensions could include a non-rex-exclusive mode or a "no king captures" variant.

[ianfab]

Thanks for the PR.

From a first glance the changes generally look good to me. However, at the current moment in a phase before a release, most PRs other than bugfixes and trivial changes are on hold. Also despite the changes being fairly isolated, the feature is also very niche, so I will need to decide on the maintenance trade-off. I will revisit it at a later point when there is a more suitable moment of a release cycle.