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/PairFiller.cs

@@ -50,6 +50,12 @@ namespace OpenNest
                     var pairParts = result.BuildParts(item.Drawing);
                     var angles = result.HullAngles;
                     var engine = new FillLinear(workArea, partSpacing);
+
+                    // Let the remainder strip try pair-based filling too.
+                    var p0 = DefaultNestEngine.BuildRotatedPattern(pairParts, 0);
+                    var p90 = DefaultNestEngine.BuildRotatedPattern(pairParts, Angle.HalfPI);
+                    engine.RemainderPatterns = new List<Pattern> { p0, p90 };
+
                     var filled = DefaultNestEngine.FillPattern(engine, pairParts, angles, workArea);
 
                     if (filled != null && filled.Count > 0)