fix(case): rename ARCP.{UnitTests,IntegrationTests} dirs to Arcp.*; fix test workflow

- git rm --cached / git add to consolidate uppercase ARCP.UnitTests/ and
  ARCP.IntegrationTests/ into mixed-case Arcp.UnitTests/ and
  Arcp.IntegrationTests/ in the git index, matching the paths already
  referenced in ARCP.slnx.  Invisible on macOS (case-insensitive FS)
  but creates a split directory on Linux that causes MSB3202.

- Update .github/workflows/test.yml:
    ARCP.sln  → ARCP.slnx  (Restore, Format check, Build steps)
    tests/ARCP.UnitTests/ARCP.UnitTests.csproj
      → tests/Arcp.UnitTests/Arcp.UnitTests.csproj  (Test step)

Author: nficano

.github/workflows/test.yml

@@ -65,17 +65,17 @@ jobs:
             ${{ runner.os }}-nuget-
 
       - name: Restore
-        run: dotnet restore ARCP.sln
+        run: dotnet restore ARCP.slnx
 
       - name: Format check
-        run: dotnet format ARCP.sln --verify-no-changes --no-restore
+        run: dotnet format ARCP.slnx --verify-no-changes --no-restore
 
       - name: Build
-        run: dotnet build ARCP.sln --configuration Release --no-restore
+        run: dotnet build ARCP.slnx --configuration Release --no-restore
 
       - name: Test
         run: >
-          dotnet test tests/ARCP.UnitTests/ARCP.UnitTests.csproj
+          dotnet test tests/Arcp.UnitTests/Arcp.UnitTests.csproj
           --configuration Release
           --no-build
           --verbosity normal