Route planning unit-merge conflicts through ConflictResolverViewModel
(OpenForPlanningAsync) and delete the old ConflictResolutionViewModel dialog.
Add active-file 3-pane reconstruction (MergeFile OursText/TheirsText/ResultText,
ActiveFile, SelectFileCommand, active-file readout) as the VM foundation for the
Rider-style editor. Seam preserved; Ui.Tests 128/128.