fix: improve fill progress reporting and engine pipeline

- Strategies now promote results to IsOverallBest when they beat the
  pipeline best, so the UI updates immediately on improvement rather
  than waiting for each phase to complete
- PlateView only updates the main view on overall-best results, fixing
  intermediate angle-sweep layouts leaking to the plate display
- Skip Row/Column strategies for rectangle parts (redundant with Linear)
- Intercept Escape key at MainForm level via ProcessCmdKey so it always
  reaches the active PlateView regardless of focus state
- Restore keyboard focus to PlateView after fill progress form closes
- Remnant engines use SelectBestFitPair for orientation-aware pair
  selection; DefaultNestEngine tries both landscape and portrait pairs
- RemnantFiller preserves more parts during topmost-part removal

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

OpenNest.Engine/Strategies/ColumnFillStrategy.cs

@@ -11,6 +11,9 @@ public class ColumnFillStrategy : IFillStrategy
 
     public List<Part> Fill(FillContext context)
     {
+        if (context.PartType == PartType.Rectangle)
+            return null;
+
         var filler = new StripeFiller(context, NestDirection.Vertical) { CompleteStripesOnly = true };
         return filler.Fill();
     }