fix(ui): offload UnifiedDiffParser.Parse off the UI thread in DiffViewerViewModel

Both call sites (LoadFilesAsync, OnDisplayedDiffChanged) ran the parser
synchronously on the dispatcher, freezing the window on a large diff. The
Files-mode parse+tree-build now runs via Task.Run behind a ParseOp
OperationStatus wired to the toolbar; the planning-mode parse is
fire-and-forget with a generation counter so a fast second DisplayedDiff
change can't write stale PlanningFiles.

tests/ClaudeDo.Ui.Tests/ViewModels/DiffViewerViewModelTests.cs

@@ -140,6 +140,33 @@ public class DiffViewerViewModelTests
         Assert.Equal("DIFF-B", vm.DisplayedDiff);
     }
 
+    [Fact]
+    public async Task Planning_SelectSubtask_PopulatesPlanningFilesAsync()
+    {
+        const string diff = "diff --git a/foo.cs b/foo.cs\n" +
+                             "--- a/foo.cs\n" +
+                             "+++ b/foo.cs\n" +
+                             "@@ -1,1 +1,1 @@\n" +
+                             "-old\n" +
+                             "+new\n";
+        var fake = new FakePlanningWorker
+        {
+            AggregateResult = new[] { new SubtaskDiffDto("s1", "First", "b1", "base1", "head1", null, diff) }
+        };
+        var vm = new DiffViewerViewModel(null!, fake, TestSettings());
+        vm.ConfigurePlanning("plan-1", "main");
+
+        await vm.LoadAsync();
+
+        // OnDisplayedDiffChanged offloads the parse via Task.Run and writes PlanningFiles back
+        // fire-and-forget, so it may not have landed yet when LoadAsync returns.
+        var deadline = DateTime.UtcNow.AddSeconds(5);
+        while (DateTime.UtcNow < deadline && vm.PlanningFiles.Count == 0) await Task.Delay(10);
+
+        var file = Assert.Single(vm.PlanningFiles);
+        Assert.Equal("foo.cs", file.Path);
+    }
+
     [Fact]
     public async Task Planning_ToggleCombined_Success_DisplaysUnifiedDiff()
     {