feat(api): add immutable GameContext type

Bundles complete game state (board, turn, castling rights, en passant, halfMoveClock, moves)
with immutable builder methods for functional state transitions.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

modules/api/src/main/scala/de/nowchess/api/board/CastlingRights.scala

@@ -0,0 +1,70 @@
+package de.nowchess.api.board
+
+/**
+ * Unified castling rights tracker for all four sides.
+ * Tracks whether castling is still available for each side and direction.
+ *
+ * @param whiteKingSide   White's king-side castling (0-0) still legally available
+ * @param whiteQueenSide  White's queen-side castling (0-0-0) still legally available
+ * @param blackKingSide   Black's king-side castling (0-0) still legally available
+ * @param blackQueenSide  Black's queen-side castling (0-0-0) still legally available
+ */
+final case class CastlingRights(
+  whiteKingSide: Boolean,
+  whiteQueenSide: Boolean,
+  blackKingSide: Boolean,
+  blackQueenSide: Boolean
+):
+  /**
+   * Check if either side has any castling rights remaining.
+   */
+  def hasAnyRights: Boolean =
+    whiteKingSide || whiteQueenSide || blackKingSide || blackQueenSide
+
+  /**
+   * Check if a specific color has any castling rights remaining.
+   */
+  def hasRights(color: Color): Boolean = color match
+    case Color.White => whiteKingSide || whiteQueenSide
+    case Color.Black => blackKingSide || blackQueenSide
+
+  /**
+   * Revoke all castling rights for a specific color.
+   */
+  def revokeColor(color: Color): CastlingRights = color match
+    case Color.White => copy(whiteKingSide = false, whiteQueenSide = false)
+    case Color.Black => copy(blackKingSide = false, blackQueenSide = false)
+
+  /**
+   * Revoke a specific castling right.
+   */
+  def revokeKingSide(color: Color): CastlingRights = color match
+    case Color.White => copy(whiteKingSide = false)
+    case Color.Black => copy(blackKingSide = false)
+
+  /**
+   * Revoke a specific castling right.
+   */
+  def revokeQueenSide(color: Color): CastlingRights = color match
+    case Color.White => copy(whiteQueenSide = false)
+    case Color.Black => copy(blackQueenSide = false)
+
+object CastlingRights:
+  /** No castling rights for any side. */
+  val None: CastlingRights = CastlingRights(
+    whiteKingSide = false,
+    whiteQueenSide = false,
+    blackKingSide = false,
+    blackQueenSide = false
+  )
+
+  /** All castling rights available. */
+  val All: CastlingRights = CastlingRights(
+    whiteKingSide = true,
+    whiteQueenSide = true,
+    blackKingSide = true,
+    blackQueenSide = true
+  )
+
+  /** Standard starting position castling rights (both sides can castle both ways). */
+  val Initial: CastlingRights = All

modules/api/src/main/scala/de/nowchess/api/game/GameContext.scala

@@ -0,0 +1,44 @@
+package de.nowchess.api.game
+
+import de.nowchess.api.board.{Board, Color, Square, CastlingRights}
+import de.nowchess.api.move.Move
+
+/** Immutable bundle of complete game state.
+ *  All state changes produce new GameContext instances.
+ */
+case class GameContext(
+  board: Board,
+  turn: Color,
+  castlingRights: CastlingRights,
+  enPassantSquare: Option[Square],
+  halfMoveClock: Int,
+  moves: List[Move]
+):
+  /** Create new context with updated board. */
+  def withBoard(newBoard: Board): GameContext = copy(board = newBoard)
+
+  /** Create new context with updated turn. */
+  def withTurn(newTurn: Color): GameContext = copy(turn = newTurn)
+
+  /** Create new context with updated castling rights. */
+  def withCastlingRights(newRights: CastlingRights): GameContext = copy(castlingRights = newRights)
+
+  /** Create new context with updated en passant square. */
+  def withEnPassantSquare(newSq: Option[Square]): GameContext = copy(enPassantSquare = newSq)
+
+  /** Create new context with updated half-move clock. */
+  def withHalfMoveClock(newClock: Int): GameContext = copy(halfMoveClock = newClock)
+
+  /** Create new context with move appended to history. */
+  def withMove(move: Move): GameContext = copy(moves = moves :+ move)
+
+object GameContext:
+  /** Initial position: white to move, all castling rights, no en passant. */
+  def initial: GameContext = GameContext(
+    board = Board.initial,
+    turn = Color.White,
+    castlingRights = CastlingRights.Initial,
+    enPassantSquare = None,
+    halfMoveClock = 0,
+    moves = List.empty
+  )