NEO-146: restore test layout section in csharp-style guide

Fix IDE0270 section that accidentally overwrote the test project layout heading.

.cursor/rules/csharp-style.md

@@ -135,7 +135,16 @@ var store = secondScope.ServiceProvider.GetRequiredService<IExampleStore>(); //
 - Use **`///` XML doc comments** on public APIs (types and members) when behavior or contracts are not obvious.
 - **Primary-constructor records:** include a **`<summary>`** plus **`<param>`** for every constructor parameter (Roslynator RCS1139 / RCS1263). Do not leave orphaned `<param>` lines without a summary on the type.
 
-## Null checks (IDE0270) (`*.Tests`)
+## Null checks (IDE0270)
+
+- Prefer **`?? throw`** over `if (x is null) { throw … }` when assigning a non-null local from a nullable expression.
+
+```csharp
+var root = FindDockerComposeRoot()
+    ?? throw new InvalidOperationException("…");
+```
+
+## Test project layout (`*.Tests`)
 
 - **Mirror the server project:** place test types under the same relative path as the production code they exercise (e.g. `NeonSprawl.Server/Game/PositionState/PositionStateApi.cs` → `NeonSprawl.Server.Tests/Game/PositionState/PositionStateApiTests.cs`).
 - Use a namespace that matches the folder tree under the test assembly root, e.g. `NeonSprawl.Server.Tests.Game.PositionState` for files in `Game/PositionState/`.