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

@@ -0,0 +1,29 @@
+package de.nowchess.rules
+
+import de.nowchess.chess.{Move, Situation, Square}
+
+/** Extension point for chess rule variants. Implement to support Chess960, etc. */
+trait RuleSet:
+  /** All pseudo-legal moves for the piece on `square` (ignores check). */
+  def candidateMoves(situation: Situation, square: Square): List[Move]
+
+  /** Legal moves for `square`: candidates that don't leave own king in check. */
+  def legalMoves(situation: Situation, square: Square): List[Move]
+
+  /** All legal moves for the side to move. */
+  def allLegalMoves(situation: Situation): List[Move]
+
+  /** True if the side to move's king is in check. */
+  def isCheck(situation: Situation): Boolean
+
+  /** True if the side to move is in check and has no legal moves. */
+  def isCheckmate(situation: Situation): Boolean
+
+  /** True if the side to move is not in check and has no legal moves. */
+  def isStalemate(situation: Situation): Boolean
+
+  /** True if neither side has enough material to checkmate. */
+  def isInsufficientMaterial(situation: Situation): Boolean
+
+  /** True if halfMoveClock >= 100 (50-move rule). */
+  def isFiftyMoveRule(situation: Situation): Boolean