Replace SetTemporaryParts/ClearTemporaryParts/AcceptTemporaryParts in all
three progress callbacks (RunAutoNest, FillPlate, FillArea) with the new
two-bucket API: SetStationaryParts for IsOverallBest updates,
SetActiveParts for transient updates, AcceptPreviewParts(parts) and
ClearPreviewParts for completion. Also removes the now-redundant
highWaterMark guards from FillPlate_Click and FillArea_Click.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>