Switch to the Avalonia 12 DevTools (WithDeveloperTools) bridge

Avalonia 12 dropped the in-process Avalonia.Diagnostics package (its last
release is 11.3.x), so AttachDevTools no longer shows a window. Move to the
AvaloniaUI.DiagnosticsSupport bridge driven by WithDeveloperTools() in
BuildAvaloniaApp -- it attaches internally, so a separate App-level
AttachDeveloperTools call would double-attach and throw. The inspector is the
AvaloniaUI.DeveloperTools global tool (avdt) connecting over the bridge.
DEBUG-only: the assets are dropped from Release builds.

Assisted-by: Claude:claude-opus-4-8:Claude Code

ILSpy/ILSpy.csproj

@@ -67,6 +67,10 @@
     <PackageReference Include="Avalonia.Desktop" />
     <PackageReference Include="Avalonia.Themes.Simple" />
     <PackageReference Include="Svg.Controls.Skia.Avalonia" />
+    <!-- Avalonia 12 DevTools: the in-app bridge half. Pairs with the AvaloniaUI.DeveloperTools
+         global tool (the inspector UI process) that connects over this bridge. DEBUG-only: the
+         assets are dropped in Release so nothing diagnostic ships. The matching WithDeveloperTools
+         and AttachDeveloperTools calls live in Program.cs / App.axaml.cs, also under #if DEBUG. -->
     <PackageReference Include="AvaloniaUI.DiagnosticsSupport">
       <IncludeAssets Condition="'$(Configuration)' != 'Debug'">None</IncludeAssets>
       <PrivateAssets Condition="'$(Configuration)' != 'Debug'">All</PrivateAssets>

ILSpy/Program.cs

@@ -33,8 +33,17 @@ namespace ILSpy
 
 		// Avalonia configuration, don't remove; also used by visual designer.
 		public static AppBuilder BuildAvaloniaApp()
-			=> AppBuilder.Configure<App>()
-				.UsePlatformDetect()
-				.LogToTrace();
+		{
+			var builder = AppBuilder.Configure<App>()
+				.UsePlatformDetect();
+#if DEBUG
+			// Enables and attaches the Avalonia 12 DevTools bridge (it calls AttachDeveloperTools
+			// internally, so no separate App-level attach is needed -- a second call throws
+			// "already attached"). The avdt global tool connects over this bridge. Gated on DEBUG
+			// because the DiagnosticsSupport assets are excluded from Release.
+			builder = builder.WithDeveloperTools();
+#endif
+			return builder.LogToTrace();
+		}
 	}
 }