refactor: NCS-22 NCS-23 reworked modules and tests (#17)

Reviewed-on: #17

modules/rule/src/main/scala/de/nowchess/rules/RuleSet.scala

@@ -0,0 +1,39 @@
+package de.nowchess.rules
+
+import de.nowchess.api.game.GameContext
+import de.nowchess.api.board.Square
+import de.nowchess.api.move.Move
+
+/** Extension point for chess rule variants (standard, Chess960, etc.).
+ *  All rule queries are stateless: given a GameContext, return the answer.
+ */
+trait RuleSet:
+  /** All pseudo-legal moves for the piece on `square` (ignores check). */
+  def candidateMoves(context: GameContext, square: Square): List[Move]
+
+  /** Legal moves for `square`: candidates that don't leave own king in check. */
+  def legalMoves(context: GameContext, square: Square): List[Move]
+
+  /** All legal moves for the side to move. */
+  def allLegalMoves(context: GameContext): List[Move]
+
+  /** True if the side to move's king is in check. */
+  def isCheck(context: GameContext): Boolean
+
+  /** True if the side to move is in check and has no legal moves. */
+  def isCheckmate(context: GameContext): Boolean
+
+  /** True if the side to move is not in check and has no legal moves. */
+  def isStalemate(context: GameContext): Boolean
+
+  /** True if neither side has enough material to checkmate. */
+  def isInsufficientMaterial(context: GameContext): Boolean
+
+  /** True if halfMoveClock >= 100 (50-move rule). */
+  def isFiftyMoveRule(context: GameContext): Boolean
+
+  /** Apply a legal move to produce the next game context.
+   *  Handles all special move types: castling, en passant, promotion.
+   *  Updates castling rights, en passant square, half-move clock, turn, and move history.
+   */
+  def applyMove(context: GameContext, move: Move): GameContext