Slice with lambda: add C# exercise projects and export

- Add SliceWithLambda.slnx + csproj scaffolding for ideas/slice-with-lambda/
  exercise/csharp (examples project + xUnit solution/test project)
- Add exercises/slice-with-lambda (C# version of the prefactored-longrange
  exercise with Buyers and SliceWithLambda projects)
- Add export/slice-with-lambda (nested git repo with exercise export)
- Ignore bin/ and obj/ build output at repo root

exercises/slice-with-lambda/tests/BuyersSolutionTests/BuyersTests.cs

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+using IceCreamBuyers;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using System.Collections.Generic;
+
+namespace IceCreamBuyers.Tests
+{
+    [TestClass]
+    public class BuyersTests
+    {
+        [TestMethod]
+        public void ShoppingListTotalUsesThePricingWePassIn()
+        {
+            // pure pricing function: everything costs 2.
+            // Anke: 2 flavours x 2 scoops x 2 = 8 — deterministic, glanceable.
+            var buyers = new Buyers();
+            var anke = new Buyer("Anke", new List<IceCream> { IceCream.Strawberry, IceCream.Vanilla });
+            Assert.AreEqual(8, buyers.ShoppingListTotal(anke, _ => 2));
+        }
+    }
+}

exercises/slice-with-lambda/tests/BuyersSolutionTests/BuyersTests.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+    <IsPackable>false</IsPackable>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.11.1" />
+    <PackageReference Include="MSTest.TestAdapter" Version="3.6.4" />
+    <PackageReference Include="MSTest.TestFramework" Version="3.6.4" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\..\solution\Buyers\Buyers.csproj" />
+  </ItemGroup>
+
+</Project>