Add exercise 3 (Buyers, C#): the pure extract-lambda rep

New in-theme classes: Buyers computes a shopping-list total, PriceList
stands for the pricing service (random surge, flaky test). One loop, one
lambda-able expression — the third example teaches the move with nothing
else in the way. Pristine repo + reference solution (extract lambda,
slide, extract method with Func<IceCream,int> parameter, pure-lambda
test) + storyboard in the established style. Mirrors the kata's C#
setup (net7.0, MSTest); needs a build check on a machine with the SDK.

ideas/split-loop-with-lambda/exercise3/csharp/test/BuyersTests.cs

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+using IceCreamBuyers;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+
+namespace IceCreamBuyers.Tests
+{
+    [TestClass]
+    public class BuyersTests
+    {
+        [TestMethod]
+        public void ShoppingListTotal()
+        {
+            // Anke: 2 flavours x 2 scoops x base prices (3 + 2) = 10.
+            // Red or flaky — why? (PriceList adds random surge pricing;
+            // it stands for the external pricing service.)
+            var buyers = new Buyers();
+            Assert.AreEqual(10, buyers.ShoppingListTotal("Anke"));
+        }
+    }
+}

ideas/split-loop-with-lambda/exercise3/csharp/test/BuyersTests.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net7.0</TargetFramework>
+    <IsPackable>false</IsPackable>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.7.1" />
+    <PackageReference Include="MSTest.TestAdapter" Version="2.1.1" />
+    <PackageReference Include="MSTest.TestFramework" Version="2.1.1" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\src\Buyers.csproj" />
+  </ItemGroup>
+
+</Project>