Approve, Submit, Reject, and Park each get their own OperationStatus so the
button locks and shows an OperationIndicator for the duration of the call;
Approve additionally scopes a MergeProgressEvent subscription to its own
task id to sharpen the label from "merging" to "verifying" mid-flight.
MergeSectionViewModel's preview refresh gets the same treatment for display
only, since it has no command button to gate.
Mika Kuns
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