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/storyboard/buyers-example.md

@@ -0,0 +1,126 @@
+# Example 3 — Buyers: extract lambda in its purest form
+
+Third rep of the Split Loop with Lambda mechanics, in C#. Example 1
+(`longRangeForecast`) had a drift trap and a dead call; the demo
+(`printSalesForecasts`) had three dependencies and grew into a role interface.
+This one has **one loop and one lambda-able expression, and nothing else** —
+the move in its purest form. Diagrams use the same
+[vocabulary as the main storyboard](storyboard.md).
+
+Kata theme: the sellers exist (`DailyForecasts` — Steve, Julie, Francis), the
+buyers don't. `Buyers` computes a buyer's shopping-list total; `PriceList`
+stands for the external pricing service. Exercise repo: `../exercise3/csharp`,
+end state: `../exercise3/solution`.
+
+---
+
+## Step 0 — one loop, one unwanted dependency
+
+**Goal:** the test is red or flaky — why? (`PriceList` adds random surge
+pricing; it stands for the external pricing service.)
+
+![](svg/b0-start.svg)
+
+**Before:**
+
+```csharp
+public int ShoppingListTotal(string buyerName)
+{
+    var buyer = ByName(buyerName);
+    var total = 0;
+    foreach (var flavour in buyer.Favourites)
+    {
+        total += buyer.ScoopsOf(flavour) * priceList.PriceFor(flavour);  // ← unwanted dependency
+    }
+    return total;
+}
+```
+
+**Checkpoint:** run the test. Flaky. What does the loop need from outside?
+*Given a flavour, what does it cost?* That question is the lambda.
+
+---
+
+## Step 1 — extract the pricing call into a lambda, and slide it
+
+**Goal:** the move, with nothing else in the way. Extract, then slide — the
+invocation stays in the loop.
+
+![](svg/b1-lambda-slide.svg)
+
+**After:**
+
+```csharp
+public int ShoppingListTotal(string buyerName)
+{
+    Func<IceCream, int> priceFor = flavour => priceList.PriceFor(flavour);  // ← SLID to the top...
+    var buyer = ByName(buyerName);
+    var total = 0;
+    foreach (var flavour in buyer.Favourites)
+    {
+        total += buyer.ScoopsOf(flavour) * priceFor(flavour);               // ← ...invocation stays here
+    }
+    return total;
+}
+```
+
+**Checkpoint:** compiles; test still flaky (the lambda still calls
+`PriceList` — that is fine). Unlike the scoring-loop example, `priceList` is a
+*field*, so the lambda slides all the way to the top. Slide, don't move.
+
+---
+
+## Step 2 — extract the loop; the lambda becomes a parameter
+
+**Goal:** select the loop, Extract Method. The loop touches no fields — the
+lambda is the only thing crossing the boundary.
+
+![](svg/b2-extract-test.svg)
+
+**After:**
+
+```csharp
+public int ShoppingListTotal(string buyerName)                           // ← no-arg overload: callers unchanged
+{
+    return ShoppingListTotal(ByName(buyerName), priceList.PriceFor);     // ← the "real" pricing, as a method group
+}
+
+public int ShoppingListTotal(Buyer buyer, Func<IceCream, int> priceFor)  // ← the seam: pricing is a parameter
+{
+    var total = 0;
+    foreach (var flavour in buyer.Favourites)
+    {
+        total += buyer.ScoopsOf(flavour) * priceFor(flavour);
+    }
+    return total;
+}
+```
+
+(The extracted loop method touches no fields; it sits in the same class as
+`ByName` for convenience. Move it to a class without `PriceList` any time —
+that is step 4 of the full pattern, unblocked.)
+
+**The test — the whole point:**
+
+```csharp
+[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));
+}
+```
+
+**Checkpoint:** green — deterministically. `PriceList` is never touched.
+
+- `grep PriceFor src/` — the extracted method has no hits.
+- C# note: the extracted method stays an instance method here because the
+  class also holds `ByName`; if you want a `static`, extract the loop only and
+  keep the lookup in the composed method.
+
+**Debrief in one line:** the loop now reads "for each favourite flavour, add
+scoops times price" — and the pricing is a question anyone can answer with one
+line of test code.