feat: NCS-11 50-move rule (#9)

Summary

  - Implements the FIDE 50-move draw rule: a player may claim a draw if no pawn move or capture has occurred in the last
   50 full moves (100 half-moves)
  - Draw is not automatic — the eligible player must claim it via a TUI menu shown at the start of their turn
  - halfMoveClock: Int is threaded through processMove and gameLoop; resets on pawn move, capture, or en passant;
  increments on all other moves

  Changes

  - GameController.scala: extended MoveResult.Moved and MoveResult.MovedInCheck with newHalfMoveClock: Int; added
  MoveResult.DrawClaimed; added halfMoveClock parameter to processMove and gameLoop; TUI menu shown when clock ≥ 100
  - Main.scala: initial gameLoop call passes halfMoveClock = 0
  - GameControllerTest.scala: updated all existing pattern matches; added 10 new tests covering clock reset, clock
  increment, draw claim, and TUI menu behaviour

  Test plan

  - processMove: 'draw' with halfMoveClock = 100 → DrawClaimed
  - processMove: 'draw' with halfMoveClock = 99 → InvalidFormat
  - Pawn move / capture / en passant → clock resets to 0
  - Quiet piece move → clock increments by 1
  - MovedInCheck carries updated clock
  - TUI menu appears when clock ≥ 100; option 1 claims draw, option 2 continues
  - No TUI menu when clock < 100
  - All 197 tests passing

Co-authored-by: LQ63 <lkhermann@web.de>
Reviewed-on: #9
Co-authored-by: Leon Hermann <lq@blackhole.local>
Co-committed-by: Leon Hermann <lq@blackhole.local>

modules/core/src/main/scala/de/nowchess/chess/engine/GameEngine.scala

@@ -90,19 +90,17 @@ class GameEngine(
         notifyObservers(event)
 
       case moveInput =>
-        // Try to parse as a move
         Parser.parseMove(moveInput) match
           case None =>
-            val event = InvalidMoveEvent(
+            notifyObservers(InvalidMoveEvent(
-              currentBoard,
+              currentBoard, currentHistory, currentTurn,
-              currentHistory,
-              currentTurn,
               s"Invalid move format '$moveInput'. Use coordinate notation, e.g. e2e4."
-            )
+            ))
-            notifyObservers(event)
-
           case Some((from, to)) =>
-            // Create a move command with current state snapshot
+            handleParsedMove(from, to, moveInput)
+  }
+
+  private def handleParsedMove(from: Square, to: Square, moveInput: String): Unit =
     val cmd = MoveCommand(
       from = from,
       to = to,
@@ -110,14 +108,11 @@ class GameEngine(
       previousHistory = Some(currentHistory),
       previousTurn = Some(currentTurn)
     )
-
-            // Execute the move through GameController
     GameController.processMove(currentBoard, currentHistory, currentTurn, moveInput) match
       case MoveResult.InvalidFormat(_) | MoveResult.NoPiece | MoveResult.WrongColor | MoveResult.IllegalMove | MoveResult.Quit =>
         handleFailedMove(moveInput)
 
       case MoveResult.Moved(newBoard, newHistory, captured, newTurn) =>
-                // Move succeeded - store result and execute through invoker
         val updatedCmd = cmd.copy(moveResult = Some(de.nowchess.chess.command.MoveResult.Successful(newBoard, newHistory, newTurn, captured)))
         invoker.execute(updatedCmd)
         updateGameState(newBoard, newHistory, newTurn)
@@ -126,7 +121,6 @@ class GameEngine(
           notifyObservers(FiftyMoveRuleAvailableEvent(currentBoard, currentHistory, currentTurn))
 
       case MoveResult.MovedInCheck(newBoard, newHistory, captured, newTurn) =>
-                // Move succeeded with check
         val updatedCmd = cmd.copy(moveResult = Some(de.nowchess.chess.command.MoveResult.Successful(newBoard, newHistory, newTurn, captured)))
         invoker.execute(updatedCmd)
         updateGameState(newBoard, newHistory, newTurn)
@@ -136,7 +130,6 @@ class GameEngine(
           notifyObservers(FiftyMoveRuleAvailableEvent(currentBoard, currentHistory, currentTurn))
 
       case MoveResult.Checkmate(winner) =>
-                // Move resulted in checkmate
         val updatedCmd = cmd.copy(moveResult = Some(de.nowchess.chess.command.MoveResult.Successful(Board.initial, GameHistory.empty, Color.White, None)))
         invoker.execute(updatedCmd)
         currentBoard = Board.initial
@@ -145,7 +138,6 @@ class GameEngine(
         notifyObservers(CheckmateEvent(currentBoard, currentHistory, currentTurn, winner))
 
       case MoveResult.Stalemate =>
-                // Move resulted in stalemate
         val updatedCmd = cmd.copy(moveResult = Some(de.nowchess.chess.command.MoveResult.Successful(Board.initial, GameHistory.empty, Color.White, None)))
         invoker.execute(updatedCmd)
         currentBoard = Board.initial
@@ -156,7 +148,6 @@ class GameEngine(
       case MoveResult.PromotionRequired(promFrom, promTo, boardBefore, histBefore, _, promotingTurn) =>
         pendingPromotion = Some(PendingPromotion(promFrom, promTo, boardBefore, histBefore, promotingTurn))
         notifyObservers(PromotionRequiredEvent(currentBoard, currentHistory, currentTurn, promFrom, promTo))
-  }
 
   /** Undo the last move. */
   def undo(): Unit = synchronized {