Add README for C# Slice-with-Lambda demo examples (ramp, table, mechanics links)

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+# Slice with Lambda — C# demo examples
+
+Three C# demo examples for the **Slice with Lambda** refactoring.
+Condensed mechanics: https://www.sammancoaching.org/refactorings/slice.html
+(where that page says mock, we use a lambda instead).
+Full Java walkthrough: `../../storyboard/storyboard.md`.
+
+## The three examples
+
+| file | guard (client state) | unwanted dependency | lambda | tail |
+|---|---|---|---|---|
+| `MetaphorFree.cs` | `if (a < 0) return 0;` | `_someDependency.mightThrowWhenPreconditionNotSatisfied(a)` | `Func<int, int> readB = a => ...` | `int c = a + b; return c * 2;` |
+| `WeatherStation.cs` | `if (_wateredToday)` | `_lightSensor.Read()` (byte) | `Func<byte> readLight = () => ...` | sunny -> `WateringDuration.Medium`, shady -> `Short` (reading > 128 => `SunExposure.Sunny`) |
+| `IceCreamShop.cs` | `if (_thawedToday)` | `_thermometer.Read()` (byte) | `Func<byte> readTemperature = () => ...` | warm -> `ThawAmount.Medium`, cold -> `Low` (temperature > 25 => `Warmth.Warm`) |
+
+In all three, the guard reads client state (a parameter or a flag the client
+owns), so it stays in the extracted method. Only the dependency call moves
+into the lambda.
+
+## The ramp
+
+`MetaphorFree.cs` is the smallest possible shape: no loop, one dependency
+call, a tail of one arithmetic step. It carries the capture-to-parameter
+lesson: the lambda takes `a` as its parameter (`readB(a)`) because a lambda
+moved up into the function head cannot capture method parameters. Its
+step-4 stub tests include a throwing stub.
+
+`WeatherStation.cs` and `IceCreamShop.cs` are same-shape domain
+repetitions — guard, one dependency call, map to enum, constant tail — so
+participants can practice extracting the lambda twice. The C# delegate
+syntax (`Func<byte>`) is worth the repetition.
+
+## Same three regions every time
+
+Each example deliberately has the same three regions:
+
+- **before**: guard clauses
+- **middle**: one dependency call
+- **after**: tail that depends on the value the dependency produced
+
+Matching the pattern: extract the dependency-using code into a lambda,
+slide the definition to the top, extract the method below it, pass the
+lambda as parameter, test with pure lambda stubs.