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/NestEngineBase.cs

@@ -56,6 +56,11 @@ namespace OpenNest
 
         protected FillPolicy BuildPolicy() => new FillPolicy(Comparer, PreferredDirection);
 
+        protected virtual BestFitResult SelectBestFitPair(List<BestFitResult> results)
+        {
+            return results.FirstOrDefault(r => r.Keep);
+        }
+
         // --- Virtual methods (side-effect-free, return parts) ---
 
         public virtual List<Part> Fill(NestItem item, Box workArea,
@@ -333,7 +338,7 @@ namespace OpenNest
 
                 var bestFits = BestFitCache.GetOrCompute(
                     item.Drawing, Plate.Size.Length, Plate.Size.Width, Plate.PartSpacing);
-                var bestFit = bestFits.FirstOrDefault(r => r.Keep);
+                var bestFit = SelectBestFitPair(bestFits);
                 if (bestFit == null) continue;
 
                 var parts = bestFit.BuildParts(item.Drawing);