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/Fill/PairFiller.cs

@@ -195,7 +195,7 @@ namespace OpenNest.Engine.Fill
                 if (pattern.Parts.Count == 0)
                     continue;
 
-                var engine = new FillLinear(workArea, partSpacing);
+                var engine = new FillLinear(workArea, partSpacing) { Label = "Pairs" };
                 foreach (var dir in new[] { NestDirection.Horizontal, NestDirection.Vertical })
                 {
                     if (!dedup.TryAdd(pattern.BoundingBox, workArea, dir))
@@ -321,7 +321,7 @@ namespace OpenNest.Engine.Fill
                 return cachedResult;
             }
 
-            var filler = new FillLinear(remnantBox, partSpacing);
+            var filler = new FillLinear(remnantBox, partSpacing) { Label = "Pairs-Remnant" };
             List<Part> parts = null;
 
             foreach (var angle in new[] { 0.0, Angle.HalfPI })