foundations for rating color advantage

Before changing the implementation to use the color advantage,
the implementer should start with adding tests to
test-kit/src/test/scala/rating/glicko/impl/RatingCalculatorWithColorAdvantageTest.scala

```sh
sbt
project testKit
~testQuick chess.rating.glicko.*
```

rating/src/main/scala/glicko/GlickoCalculator.scala

@@ -9,10 +9,11 @@ import scala.util.Try
 /* Purely functional interface hiding the mutable implementation */
 final class GlickoCalculator(
     tau: Tau = Tau.default,
-    ratingPeriodsPerDay: RatingPeriodsPerDay = RatingPeriodsPerDay.default
+    ratingPeriodsPerDay: RatingPeriodsPerDay = RatingPeriodsPerDay.default,
+    colorAdvantage: ColorAdvantage = ColorAdvantage.zero
 ):
 
-  private val calculator = new impl.RatingCalculator(tau, ratingPeriodsPerDay)
+  private val calculator = new impl.RatingCalculator(tau, ratingPeriodsPerDay, colorAdvantage)
 
   // Simpler use case: a single game
   def computeGame(game: Game, skipDeviationIncrease: Boolean = false): Try[ByColor[Player]] =

rating/src/main/scala/glicko/impl/Rating.scala

@@ -16,11 +16,14 @@ final private[glicko] class Rating(
   private[impl] var workingRatingDeviation: Double = scala.compiletime.uninitialized
   private[impl] var workingVolatility: Double      = scala.compiletime.uninitialized
 
-  /** Return the average skill value of the player scaled down to the scale used by the algorithm's internal
-    * workings.
-    */
+  /** Return the average skill value of the player 
+   *  scaled down to the scale used by the algorithm's internal workings.
+   */
   private[impl] def getGlicko2Rating: Double = convertRatingToGlicko2Scale(this.rating)
 
+  private[impl] def getGlicko2RatingWithAdvantage(advantage: ColorAdvantage): Double =
+    convertRatingToGlicko2Scale(this.rating + advantage.value)
+
   /** Set the average skill value, taking in a value in Glicko2 scale.
     */
   private[impl] def setGlicko2Rating(r: Double) =

rating/src/main/scala/glicko/impl/RatingCalculator.scala

@@ -24,7 +24,8 @@ private object RatingCalculator:
 
 final private[glicko] class RatingCalculator(
     tau: Tau = Tau.default,
-    ratingPeriodsPerDay: RatingPeriodsPerDay = RatingPeriodsPerDay.default
+    ratingPeriodsPerDay: RatingPeriodsPerDay = RatingPeriodsPerDay.default,
+    colorAdvantage: ColorAdvantage = ColorAdvantage.zero
 ):
 
   import RatingCalculator.*

rating/src/main/scala/glicko/model.scala

@@ -48,3 +48,10 @@ object Tau extends OpaqueDouble[Tau]:
 opaque type RatingPeriodsPerDay = Double
 object RatingPeriodsPerDay extends OpaqueDouble[RatingPeriodsPerDay]:
   val default: RatingPeriodsPerDay = 0d
+
+opaque type ColorAdvantage = Double
+object ColorAdvantage extends OpaqueDouble[ColorAdvantage]:
+  val zero: ColorAdvantage                                 = 0d
+  val standard: ColorAdvantage                             = 7.786d
+  val crazyhouse: ColorAdvantage                           = 15.171d
+  extension (c: ColorAdvantage) def negate: ColorAdvantage = -c

test-kit/src/test/scala/rating/glicko/GlickoCalculatorWithColorAdvantageTest.scala

@@ -0,0 +1,14 @@
+package chess.rating.glicko
+
+import cats.syntax.all.*
+import munit.ScalaCheckSuite
+
+class RatingCalculatorWithColorAdvantageTest extends ScalaCheckSuite with chess.MunitExtensions:
+
+  val smallAdvantage = GlickoCalculator(
+    Tau.default,
+    RatingPeriodsPerDay.default,
+    ColorAdvantage(7d)
+  )
+
+  // test that the rating calculator correctly applies the color advantage