Defer Title and SyntaxExtension until decompile actually lands

A cancelled decompile used to leave the tab title pointing at the new
node while the editor still showed the previous node's output. Move
Title and SyntaxExtension into the same UI-thread post that publishes
the rendered text, foldings, and highlighting model — so all five land
together when the decompile succeeds, and stay at the previous values
on cancel.

Assisted-by: Claude:claude-opus-4-7:Claude Code

ILSpy/TextView/DecompilerTabPageModel.cs

@@ -115,8 +115,7 @@ namespace ILSpy.TextView
 			}
 
 			var nodeTitle = node.Text?.ToString() ?? "(unnamed)";
-			Title = nodeTitle;
-			SyntaxExtension = language.FileExtension;
+			var newSyntaxExtension = language.FileExtension;
 			IsDecompiling = true;
 
 			try
@@ -149,6 +148,8 @@ namespace ILSpy.TextView
 				var model = output.HighlightingModel;
 				var collectedFoldings = output.Foldings;
 				await Dispatcher.UIThread.InvokeAsync(() => {
+					Title = nodeTitle;
+					SyntaxExtension = newSyntaxExtension;
 					HighlightingModel = model;
 					Foldings = collectedFoldings;
 					Text = rendered;