Make pieces sendable

Model/Sources/Model/Piece.swift

@@ -1,4 +1,4 @@
-public struct Piece {
+public struct Piece : Sendable {
     public let owner: Player
     
     // Required for public visibility

Model/Sources/Model/Player.swift

@@ -1,3 +1,3 @@
-public enum Player: CaseIterable {
+public enum Player: CaseIterable, Sendable {
     case A, B
 }