fix: correct 50-move rule threshold to 100 half-moves (FIDE-compliant)

The halfMoveClock counts plies (half-moves). The FIDE 50-move rule requires
50 moves by each side = 100 plies, not 50. Changed both the processMove
and gameLoop checks from >= 50 to >= 100, and updated all tests accordingly.

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

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

@@ -36,7 +36,7 @@ object GameController:
       case "quit" | "q" =>
         MoveResult.Quit
       case "draw" =>
-        if halfMoveClock >= 50 then MoveResult.DrawClaimed
+        if halfMoveClock >= 100 then MoveResult.DrawClaimed
         else MoveResult.InvalidFormat("draw")
       case trimmed =>
         Parser.parseMove(trimmed) match
@@ -80,7 +80,7 @@ object GameController:
     println()
     print(Renderer.render(board))
     val input =
-      if halfMoveClock >= 50 then
+      if halfMoveClock >= 100 then
         println(s"[50-move rule] ${turn.label} may claim a draw, or continue playing.")
         println("  1. Claim draw")
         println("  2. Continue")

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

@@ -404,19 +404,19 @@ class GameControllerTest extends AnyFunSuite with Matchers:
 
   // ──── processMove: 50-move rule draw claim ───────────────────────────────
 
-  test("processMove: 'draw' with halfMoveClock = 50 returns DrawClaimed"):
+  test("processMove: 'draw' with halfMoveClock = 100 returns DrawClaimed"):
     val b = Board(Map(
       sq(File.E, Rank.R1) -> Piece.WhiteKing,
       sq(File.E, Rank.R8) -> Piece.BlackKing
     ))
-    GameController.processMove(b, GameHistory.empty, Color.White, 50, "draw") shouldBe MoveResult.DrawClaimed
+    GameController.processMove(b, GameHistory.empty, Color.White, 100, "draw") shouldBe MoveResult.DrawClaimed
 
-  test("processMove: 'draw' with halfMoveClock = 49 returns InvalidFormat"):
+  test("processMove: 'draw' with halfMoveClock = 99 returns InvalidFormat"):
     val b = Board(Map(
       sq(File.E, Rank.R1) -> Piece.WhiteKing,
       sq(File.E, Rank.R8) -> Piece.BlackKing
     ))
-    GameController.processMove(b, GameHistory.empty, Color.White, 49, "draw") shouldBe MoveResult.InvalidFormat("draw")
+    GameController.processMove(b, GameHistory.empty, Color.White, 99, "draw") shouldBe MoveResult.InvalidFormat("draw")
 
   // ──── processMove: halfMoveClock update ──────────────────────────────────
 
@@ -470,30 +470,30 @@ class GameControllerTest extends AnyFunSuite with Matchers:
 
   // ──── gameLoop: 50-move rule menu ────────────────────────────────────────
 
-  test("gameLoop: shows 50-move rule menu when halfMoveClock >= 50 and draw claimed"):
+  test("gameLoop: shows 50-move rule menu when halfMoveClock >= 100 and draw claimed"):
     val b = Board(Map(
       sq(File.E, Rank.R1) -> Piece.WhiteKing,
       sq(File.E, Rank.R8) -> Piece.BlackKing
     ))
     val output = captureOutput:
       withInput("1\nquit\n"):
-        GameController.gameLoop(b, GameHistory.empty, Color.White, 50)
+        GameController.gameLoop(b, GameHistory.empty, Color.White, 100)
     output should include("50-move rule")
     output should include("Draw claimed by 50-move rule.")
 
-  test("gameLoop: shows 50-move rule menu when halfMoveClock >= 50 and player continues"):
+  test("gameLoop: shows 50-move rule menu when halfMoveClock >= 100 and player continues"):
     val b = Board(Map(
       sq(File.E, Rank.R1) -> Piece.WhiteKing,
       sq(File.E, Rank.R8) -> Piece.BlackKing
     ))
     val output = captureOutput:
       withInput("2\nquit\n"):
-        GameController.gameLoop(b, GameHistory.empty, Color.White, 50)
+        GameController.gameLoop(b, GameHistory.empty, Color.White, 100)
     output should include("50-move rule")
     output should include("White's turn")
 
-  test("gameLoop: no 50-move rule menu when halfMoveClock < 50"):
+  test("gameLoop: no 50-move rule menu when halfMoveClock < 100"):
     val output = captureOutput:
       withInput("quit\n"):
-        GameController.gameLoop(Board.initial, GameHistory.empty, Color.White, 49)
+        GameController.gameLoop(Board.initial, GameHistory.empty, Color.White, 99)
     output should not include "50-move rule"