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/Fill/RemnantFiller.cs

@@ -106,7 +106,7 @@ namespace OpenNest.Engine.Fill
                 // rectangular obstacle boundary. Without this, gaps between
                 // individual bounding boxes cause the next drawing to fill
                 // into inter-row spaces, producing an interleaved layout.
-                if (placed.Count > 1)
+                if (placed.Count > 2)
                     RemoveTopmostPart(placed);
 
                 allParts.AddRange(placed);