fix(engine): fix FillExtents competition and vertical gap bugs

- FillExtents.Fill reported progress internally which overwrote the UI's
  temporary parts even when a better result (e.g. Pairs with 70 parts)
  won the competition. Added final ReportProgress call in FindBestFill
  and Fill(groupParts) to ensure the UI always shows the actual winner.

- FillExtents vertical copy distance clamp (Math.Max with pairHeight +
  spacing) prevented geometry-aware compaction from ever occurring,
  causing visible gaps between rows. Boundaries are already inflated by
  halfSpacing so the calculated distance is correct; only fall back to
  bounding-box distance on non-positive results.

- PairFiller now sets RemainderPatterns on FillLinear so remainder strips
  get pair-based filling instead of only individual parts (+1 part in
  tight layouts).

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

OpenNest.Engine/DefaultNestEngine.cs

@@ -176,6 +176,13 @@ namespace OpenNest
                 }
             }
 
+            // Always report the final winner so the UI's temporary parts
+            // match the returned result.
+            var winPhase = PhaseResults.Count > 0
+                ? PhaseResults.OrderByDescending(r => r.PartCount).First().Phase
+                : NestPhase.Linear;
+            ReportProgress(progress, winPhase, PlateNumber, best, workArea, BuildProgressSummary());
+
             return best ?? new List<Part>();
         }
 
@@ -320,6 +327,11 @@ namespace OpenNest
                 Debug.WriteLine("[FindBestFill] Cancelled, returning current best");
             }
 
+            // Always report the final winner so the UI's temporary parts
+            // match the returned result (sub-phases may have reported their
+            // own intermediate results via progress).
+            ReportProgress(progress, WinnerPhase, PlateNumber, best, workArea, BuildProgressSummary());
+
             return best ?? new List<Part>();
         }