DiscountedOrder domain type + DiscountCalculator (half-up, strict barrier) with unit tests

yak: discountedorder-discountcalculator-with-unit-tests-zrtn

src/After/DiscountedOrder.cs

@@ -0,0 +1,26 @@
+namespace After;
+
+/// <summary>
+/// The result of discounting an <see cref="Order"/>: a separate domain type,
+/// not a modified order. The discount is derived data — computed on demand,
+/// correlated to its order by <see cref="OrderId"/>. What a caller does with
+/// it (and what of it ever reaches <see cref="OrderDto"/> or
+/// <see cref="OrderData"/>) is deliberately left to the caller.
+/// </summary>
+public sealed record DiscountedOrder
+{
+    /// <summary>Primary key of the discounted <see cref="Order"/>.</summary>
+    public Guid OrderId { get; init; }
+
+    /// <summary>The order's total in cents, as read from it (the untouched input).</summary>
+    public int TotalInCents { get; init; }
+
+    /// <summary>
+    /// The percentage actually applied, as an integer percent. 0 when the
+    /// order did not pass the discount barrier.
+    /// </summary>
+    public int DiscountPercentage { get; init; }
+
+    /// <summary>The total after the discount, in cents.</summary>
+    public int DiscountedTotalInCents { get; init; }
+}