Relocate test project: test/yavscTests -> src/Yavsc.Org.Tests

Move the integration test project from the top-level test/ directory into
src/ alongside the projects it tests. Rename the project (and folder) to
Yavsc.Org.Tests to match .NET conventions and reflect that it tests the
Org runtime primarily.

Path changes:
- test/yavscTests/yavscTests.csproj -> src/Yavsc.Org.Tests/Yavsc.Org.Tests.csproj
- All .cs / .json / .resx files moved to their new location
- PostItViewModelTests moved out to the dedicated src/PostIt.Tests project
  (it was unrelated to Org testing)

Build adjustments:
- <ProjectReference> paths shortened (..\..\src\X -> ..\X)
- PostIt project reference removed (covered by its own test project)
- <OutputType>exe added (required by xunit.v3)
- xunit.v3.common and xunit.v3.extensibility.core added to package versions

Solution + sln:
- yavsc.sln Project Name updated to 'Yavsc.Org.Tests' and path updated
- GUID preserved so existing build configs stay valid

Static web assets:
- The CopyStaticWebAssetsManifest target was hard-coding the destination
  filename to 'testhost.staticwebassets.endpoints.json', which worked
  when the assembly was named 'yavscTests'. Now that the assembly name
  is 'Yavsc.Org.Tests', ASP.NET Core's MapStaticAssets() looks for
  'Yavsc.Org.Tests.staticwebassets.endpoints.json' (entry-assembly-based
  resolution). Use $(MSBuildProjectName) so the copy target stays
  correct under any future rename.

src/Yavsc.Org.Tests/NonRegression/BillingServiceTests.cs

@@ -0,0 +1,48 @@
+using Microsoft.EntityFrameworkCore;
+using Xunit;
+using Yavsc;
+using Yavsc.Abstract.Workflow;
+using Yavsc.Helpers;
+using Yavsc.Models;
+using Yavsc.Models.Billing;
+using Yavsc.Models.Haircut;
+using Yavsc.Services;
+
+namespace yavscTests
+{
+    [Trait("regression", "II")]
+    public class BillingServiceTests
+    {
+        [Fact]
+        public void ConfigureBillingService_CanBeCalledTwiceWithoutThrowing()
+        {
+            // First initialization should populate the billing registry.
+            WorkflowHelpers.ConfigureBillingService();
+
+            int firstBillingCount = BillingService.Billing.Count;
+            int firstSettingsCount = BillingService.UserSettings.Count;
+            int firstProfileTypesCount = Config.ProfileTypes.Count;
+
+            // Second call should be idempotent and not throw.
+            WorkflowHelpers.ConfigureBillingService();
+
+            Assert.Equal(firstBillingCount, BillingService.Billing.Count);
+            Assert.Equal(firstSettingsCount, BillingService.UserSettings.Count);
+            Assert.Equal(firstProfileTypesCount, Config.ProfileTypes.Count);
+        }
+
+        [Fact]
+        public void RegisterBilling_DuplicateRegistrationThrowsInvalidOperationException()
+        {
+            WorkflowHelpers.ConfigureBillingService();
+
+            var firstRegistrar = new Func<ApplicationDbContext, long, IDecidableQuery>((db, id) =>
+                db.HairCutQueries.Include(q => q.Prestation).Include(q => q.Regularisation).Single(q => q.Id == id));
+
+            const string testCode = "Brush";
+            
+            Assert.Throws<InvalidOperationException>(() =>
+                WorkflowHelpers.RegisterBilling<HairCutQuery>(testCode, firstRegistrar));
+        }
+    }
+}