fix: add overlap safety check and diagnostics to FillGrid Step 2

FillGrid had no overlap check after perpendicular tiling of the row
pattern (Step 2), unlike Step 1 which had one. When geometry-aware
FindPatternCopyDistance underestimated row spacing, overlapping parts
were returned unchecked.

Changes:
- Make FillLinear.HasOverlappingParts shape-aware (bbox pre-filter +
  Part.Intersects) instead of bbox-only, preventing false positives on
  interlocking pairs while catching real overlaps
- Add missing overlap safety check after Step 2 perpendicular tiling
  with bbox fallback
- Add diagnostic Debug.WriteLine logging when overlap fallback triggers,
  including engine label, step, direction, work area, spacing, pattern
  details, and overlapping part locations/rotations for reproduction
- Add FillLinear.Label property set at all callsites for log traceability
- Refactor LinearFillStrategy and ExtentsFillStrategy to use shared
  FillHelpers.BestOverAngles helper for angle-sweep logic

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

OpenNest.Engine/DefaultNestEngine.cs

@@ -94,7 +94,7 @@ namespace OpenNest
 
             // Multi-part group: linear pattern fill only.
             PhaseResults.Clear();
-            var engine = new FillLinear(workArea, Plate.PartSpacing);
+            var engine = new FillLinear(workArea, Plate.PartSpacing) { Label = "GroupPattern" };
             var angles = RotationAnalysis.FindHullEdgeAngles(groupParts);
             var best = FillHelpers.FillPattern(engine, groupParts, angles, workArea, Comparer);
             PhaseResults.Add(new PhaseResult(NestPhase.Linear, best?.Count ?? 0, 0));