ParseOp only covered LoadFilesAsync; OnDisplayedDiffChanged's parse (driven
by ToggleCombinedAsync and OnSelectedSubtaskChanged) had no visible
feedback even though the offload was already correct. Both call sites now
go through one ParseOffUiThreadAsync helper (Begin on the UI thread before
Task.Run, End via `using` regardless of a stale generation), so the
already-visible toolbar indicator covers the planning path without
duplicating the begin/offload/end ceremony.