feat(core): core dependecy to rules

Added communication between core and rules via REST

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

@@ -0,0 +1,41 @@
+package de.nowchess.api.rules
+
+import de.nowchess.api.board.Square
+import de.nowchess.api.game.GameContext
+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
+
+  /** True if the same position has occurred 3 times (including current position). */
+  def isThreefoldRepetition(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