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/global.json

@@ -0,0 +1,7 @@
+{
+  "sdk": {
+    "version": "7.0.0",
+    "rollForward": "latestMajor",
+    "allowPrerelease": true
+  }
+}