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/svg/b1-lambda-slide.svg

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+    <marker id="arrP" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="7" markerHeight="7" orient="auto-start-reverse">
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+  <rect width="760" height="460" fill="#ffffff"/>
+  <text x="24" y="34" font-size="20" font-weight="bold" fill="#0f172a">Step 1 — extract the pricing call into a lambda, and slide it</text>
+  <text x="24" y="56" font-size="13" fill="#9333ea" font-weight="bold">One expression, nothing else in the way. priceList is a field, so the lambda slides all the way to the top.</text>
+
+  <!-- method -->
+  <rect x="60" y="74" width="400" height="330" rx="10" fill="#f8fafc" stroke="#64748b" stroke-width="1.5"/>
+  <text x="76" y="98" font-size="14" font-weight="bold" fill="#0f172a">ShoppingListTotal(buyerName)</text>
+
+  <!-- lambda at top -->
+  <rect x="80" y="110" width="360" height="82" rx="8" fill="#faf5ff" stroke="#9333ea" stroke-width="1.5"/>
+  <text x="94" y="136" font-size="12" font-weight="bold" fill="#9333ea">λ priceFor</text>
+  <text x="94" y="162" font-size="11" font-family="Menlo, Consolas, monospace" fill="#dc2626">Func&lt;IceCream,int&gt; =</text>
+  <text x="94" y="180" font-size="11" font-family="Menlo, Consolas, monospace" fill="#dc2626">  flavour =&gt; priceList.PriceFor(flavour);</text>
+
+  <text x="80" y="200" font-size="11" font-family="Menlo, Consolas, monospace" fill="#1e1b4b">buyer = ByName(buyerName)</text>
+
+  <!-- loop -->
+  <rect x="80" y="214" width="360" height="120" rx="8" fill="#eef2ff" stroke="#818cf8"/>
+  <text x="94" y="240" font-size="11" font-family="Menlo, Consolas, monospace" fill="#1e1b4b">foreach (var flavour : buyer.Favourites)</text>
+  <text x="94" y="270" font-size="11" font-family="Menlo, Consolas, monospace" fill="#1e1b4b">total += buyer.ScoopsOf(flavour) *</text>
+  <text x="94" y="290" font-size="11" font-family="Menlo, Consolas, monospace" font-weight="bold" fill="#6b21a8">  priceFor(flavour);</text>
+  <text x="94" y="318" font-size="10" fill="#64748b">invocation stays exactly where the code was</text>
+
+  <!-- ghost -->
+  <rect x="94" y="352" width="330" height="26" rx="6" fill="none" stroke="#9333ea" stroke-width="1" stroke-dasharray="4 3"/>
+  <text x="106" y="370" font-size="10" fill="#9333ea" font-style="italic">λ used to be defined right here</text>
+
+  <text x="80" y="398" font-size="11" font-family="Menlo, Consolas, monospace" fill="#1e1b4b">return total;</text>
+
+  <!-- slide arrow -->
+  <line x1="260" y1="350" x2="260" y2="196" stroke="#9333ea" stroke-width="1.5" stroke-dasharray="5 4" marker-end="url(#arrP)"/>
+  <text x="270" y="205" font-size="12" font-weight="bold" fill="#9333ea">slide</text>
+
+  <!-- PriceList -->
+  <rect x="580" y="130" width="150" height="90" rx="8" fill="#fef2f2" stroke="#dc2626" stroke-width="1.5"/>
+  <text x="655" y="160" font-size="14" font-weight="bold" fill="#dc2626" text-anchor="middle">PriceList</text>
+  <text x="655" y="180" font-size="11" fill="#7f1d1d" text-anchor="middle">still called —</text>
+  <text x="655" y="196" font-size="11" fill="#7f1d1d" text-anchor="middle">but only via the λ now</text>
+
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