Fix GRN pricing

Distribution/Features/Grn/GrnPricing.cs

@@ -0,0 +1,32 @@
+namespace Distribution.Features.Grn;
+
+/// <summary>
+/// GRN costing. Per line: gross = accepted qty x unit cost; discount = gross x disc%;
+/// VAT = (gross - discount) x VAT%; total = gross - discount + VAT. Rejected units are never costed.
+/// Each amount is rounded to 2 dp per line so the GRN totals equal the sum of what the lines show.
+/// </summary>
+public static class GrnPricing
+{
+    public static decimal Round(decimal value) => Math.Round(value, 2, MidpointRounding.AwayFromZero);
+
+    public static (decimal Gross, decimal Discount, decimal Vat, decimal Total) Line(
+        int acceptedQty, decimal unitCost, decimal discountPercent, decimal vatPercent)
+    {
+        var gross = Round(acceptedQty * unitCost);
+        var discount = Round(gross * discountPercent / 100m);
+        var vat = Round((gross - discount) * vatPercent / 100m);
+        return (gross, discount, vat, gross - discount + vat);
+    }
+
+    public static void ValidatePercent(decimal value, string label)
+    {
+        if (value < 0 || value > 100)
+            throw new InvalidOperationException($"{label} must be between 0 and 100.");
+    }
+
+    public static void ValidateCost(decimal value, string label)
+    {
+        if (value < 0)
+            throw new InvalidOperationException($"{label} cannot be negative.");
+    }
+}