fix: update move validation to check for king safety (#13)

Reviewed-on: #13

.claude/agents/test-writer.md

@@ -20,5 +20,4 @@ When invoked BEFORE scala-implementer (no implementation exists yet):
 
 When invoked AFTER scala-implementer (implementation exists):
   Run python3 jacoco-reporter/jacoco_coverage_gaps.py modules/{service-name}/build/reports/jacoco/test/jacocoTestReport.xml --output agent
-  Use the jacoco-coverage-gaps skill — close coverage gaps revealed by the report.
   To regenerate the report run the tests first.

modules/core/src/main/scala/de/nowchess/chess/controller/GameController.scala

@@ -75,7 +75,7 @@ object GameController:
       case None                              => MoveResult.NoPiece
       case Some(piece) if piece.color != turn => MoveResult.WrongColor
       case Some(_) =>
-        if !MoveValidator.isLegal(board, history, from, to) then MoveResult.IllegalMove
+        if !GameRules.legalMoves(board, history, turn).contains(from -> to) then MoveResult.IllegalMove
         else if MoveValidator.isPromotionMove(board, from, to) then
           MoveResult.PromotionRequired(from, to, board, history, board.pieceAt(to), turn)
         else applyNormalMove(board, history, turn, from, to)

modules/core/src/test/scala/de/nowchess/chess/controller/GameControllerTest.scala

@@ -43,6 +43,30 @@ class GameControllerTest extends AnyFunSuite with Matchers:
     // White pawn at E2 cannot jump three squares to E5
     processMove(Board.initial, GameHistory.empty, Color.White, "e2e5") shouldBe MoveResult.IllegalMove
 
+  test("processMove: move that leaves own king in check returns IllegalMove"):
+    // White King E1 is in check from Black Rook E8. Moving the D2 pawn is
+    // geometrically legal but does not resolve the check — must be rejected.
+    val b = Board(Map(
+      sq(File.E, Rank.R1) -> Piece.WhiteKing,
+      sq(File.D, Rank.R2) -> Piece.WhitePawn,
+      sq(File.E, Rank.R8) -> Piece.BlackRook,
+      sq(File.A, Rank.R8) -> Piece.BlackKing
+    ))
+    processMove(b, GameHistory.empty, Color.White, "d2d4") shouldBe MoveResult.IllegalMove
+
+  test("processMove: move that resolves check is allowed"):
+    // White King E1 is in check from Black Rook E8 along the E-file.
+    // White Rook A5 interposes at E5 — resolves the check, no new check on Black King A8.
+    val b = Board(Map(
+      sq(File.E, Rank.R1) -> Piece.WhiteKing,
+      sq(File.A, Rank.R5) -> Piece.WhiteRook,
+      sq(File.E, Rank.R8) -> Piece.BlackRook,
+      sq(File.A, Rank.R8) -> Piece.BlackKing
+    ))
+    processMove(b, GameHistory.empty, Color.White, "a5e5") match
+      case _: MoveResult.Moved => succeed
+      case other => fail(s"Expected Moved, got $other")
+
   test("processMove: legal pawn move returns Moved with updated board and flipped turn"):
     processMove(Board.initial, GameHistory.empty, Color.White, "e2e4") match
       case MoveResult.Moved(newBoard, newHistory, captured, newTurn) =>