WIP: Almost there!

Refactored tests and code to align with the modern instance-based
application model. Key changes include:

- Disabled Sixel rendering in `OutputBase.cs` due to dependency on
  legacy static `Application` object.
- Hardcoded `force16Colors` to `false` in `WindowsOutput.cs` with a
  `BUGBUG` note.
- Updated `ApplicationImplTests` to use `ApplicationImpl.SetInstance`
  and return `ApplicationImpl.Instance`.
- Refactored `ApplicationModelFencingTests` to use `Application.Create()`
  and added `ResetModelUsageTracking()` for model switching.
- Removed legacy `DriverTests` and reintroduced updated versions with
  cross-platform driver tests.
- Reverted `ArrangementTests` and `ShortcutTests` to use legacy static
  `ApplicationImpl.Instance`.
- Reintroduced driver tests in `DriverTests.cs` with modern `Application.Create()`
  and added `TestTop` for driver content verification.
- General cleanup, including removal of outdated code and addition of
  `BUGBUG` notes for temporary workarounds.

Tests/UnitTests/Application/ApplicationModelFencingTests.cs

@@ -17,7 +17,7 @@ public class ApplicationModelFencingTests
     public void Create_ThenInstanceAccess_ThrowsInvalidOperationException ()
     {
         // Create a modern instance-based application
-        IApplication app = ApplicationImpl.Instance; // Force legacy
+        IApplication app = Application.Create ();
         app.Init ("fake");
 
         // Attempting to initialize using the legacy static model should throw
@@ -31,6 +31,8 @@ public class ApplicationModelFencingTests
 
         // Clean up
         app.Shutdown ();
+        ApplicationImpl.ResetModelUsageTracking ();
+
     }
 
     [Fact]
@@ -43,7 +45,7 @@ public class ApplicationModelFencingTests
         // Attempting to create and initialize with modern instance-based model should throw
         InvalidOperationException ex = Assert.Throws<InvalidOperationException> (() =>
         {
-            IApplication app = ApplicationImpl.Instance; // Force legacy
+            IApplication app = Application.Create ();
             app.Init ("fake");
         });
 
@@ -52,6 +54,7 @@ public class ApplicationModelFencingTests
 
         // Clean up
         staticInstance.Shutdown ();
+        ApplicationImpl.ResetModelUsageTracking ();
     }
 
     [Fact]
@@ -72,13 +75,14 @@ public class ApplicationModelFencingTests
 
         // Clean up
         staticInstance.Shutdown ();
+        ApplicationImpl.ResetModelUsageTracking ();
     }
 
     [Fact]
     public void Create_ThenInit_ThrowsInvalidOperationException ()
     {
         // Create a modern instance-based application
-        IApplication app = ApplicationImpl.Instance; // Force legacy
+        IApplication app = Application.Create ();
         app.Init ("fake");
 
         // Attempting to initialize using the legacy static model should throw
@@ -92,6 +96,7 @@ public class ApplicationModelFencingTests
 
         // Clean up
         app.Shutdown ();
+        ApplicationImpl.ResetModelUsageTracking ();
     }
 
     [Fact]
@@ -110,6 +115,7 @@ public class ApplicationModelFencingTests
         app1.Shutdown ();
         app2.Shutdown ();
         app3.Shutdown ();
+        ApplicationImpl.ResetModelUsageTracking ();
     }
 
     [Fact]
@@ -126,6 +132,7 @@ public class ApplicationModelFencingTests
 
         // Clean up
         instance1.Shutdown ();
+        ApplicationImpl.ResetModelUsageTracking ();
     }
 
     [Fact]
@@ -150,5 +157,6 @@ public class ApplicationModelFencingTests
         IApplication app2 = Application.Create ();
         Assert.NotNull (app2);
         app2.Shutdown ();
+        ApplicationImpl.ResetModelUsageTracking ();
     }
 }