feat: include castling moves in GameRules.legalMoves

Switch legalMoves to the context-aware MoveValidator.legalTargets(ctx, from)
so castling destinations are included, and simulate castle moves via withCastle
when filtering for self-check.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

modules/core/src/main/scala/de/nowchess/chess/logic/GameRules.scala

@@ -24,9 +24,13 @@ object GameRules:
     ctx.board.pieces
       .collect { case (from, piece) if piece.color == color => from }
       .flatMap { from =>
-        MoveValidator.legalTargets(ctx.board, from)
+        MoveValidator.legalTargets(ctx, from)           // context-aware: includes castling
           .filter { to =>
-            val (newBoard, _) = ctx.board.withMove(from, to)
+            val newBoard =
+              if MoveValidator.isCastle(ctx.board, from, to) then
+                ctx.board.withCastle(color, MoveValidator.castleSide(from, to))
+              else
+                ctx.board.withMove(from, to)._1
             !isInCheck(newBoard, color)
           }
           .map(to => from -> to)

modules/core/src/test/scala/de/nowchess/chess/logic/GameRulesTest.scala

@@ -1,6 +1,7 @@
 package de.nowchess.chess.logic
 
 import de.nowchess.api.board.*
+import de.nowchess.api.game.CastlingRights
 import de.nowchess.chess.logic.GameContext
 import org.scalatest.funsuite.AnyFunSuite
 import org.scalatest.matchers.should.Matchers
@@ -88,3 +89,28 @@ class GameRulesTest extends AnyFunSuite with Matchers:
 
   test("gameStatus: normal starting position returns Normal"):
     GameRules.gameStatus(Board.initial, Color.White) shouldBe PositionStatus.Normal
+
+  test("legalMoves: includes castling destination when available"):
+    val c = GameContext(
+      board = board(
+        sq(File.E, Rank.R1) -> Piece.WhiteKing,
+        sq(File.H, Rank.R1) -> Piece.WhiteRook,
+        sq(File.H, Rank.R8) -> Piece.BlackKing
+      ),
+      whiteCastling = CastlingRights.Both,
+      blackCastling = CastlingRights.None
+    )
+    GameRules.legalMoves(c, Color.White) should contain(sq(File.E, Rank.R1) -> sq(File.G, Rank.R1))
+
+  test("legalMoves: excludes castling when king is in check"):
+    val c = GameContext(
+      board = board(
+        sq(File.E, Rank.R1) -> Piece.WhiteKing,
+        sq(File.H, Rank.R1) -> Piece.WhiteRook,
+        sq(File.E, Rank.R8) -> Piece.BlackRook,
+        sq(File.A, Rank.R8) -> Piece.BlackKing
+      ),
+      whiteCastling = CastlingRights.Both,
+      blackCastling = CastlingRights.None
+    )
+    GameRules.legalMoves(c, Color.White) should not contain (sq(File.E, Rank.R1) -> sq(File.G, Rank.R1))