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/Forms/PatternTileForm.cs

@@ -220,7 +220,7 @@ namespace OpenNest.Forms
                 return;
 
             var workArea = new Box(0, 0, plateSize.Length, plateSize.Width);
-            var filler = new FillLinear(workArea, PartSpacing);
+            var filler = new FillLinear(workArea, PartSpacing) { Label = "PatternTile-H" };
 
             var hParts = filler.Fill(pattern, NestDirection.Horizontal);
             foreach (var part in hParts)
@@ -228,7 +228,7 @@ namespace OpenNest.Forms
             hLabel.Text = $"Horizontal — {hParts.Count} parts";
             hPreview.ZoomToFit();
 
-            var vFiller = new FillLinear(workArea, PartSpacing);
+            var vFiller = new FillLinear(workArea, PartSpacing) { Label = "PatternTile-V" };
             var vParts = vFiller.Fill(pattern, NestDirection.Vertical);
             foreach (var part in vParts)
                 vPreview.Plate.Parts.Add(part);
@@ -328,7 +328,7 @@ namespace OpenNest.Forms
             if (pattern == null)
                 return;
 
-            var filler = new FillLinear(new Box(0, 0, plateSize.Length, plateSize.Width), PartSpacing);
+            var filler = new FillLinear(new Box(0, 0, plateSize.Length, plateSize.Width), PartSpacing) { Label = "PatternTile-Apply" };
             var tiledParts = filler.Fill(pattern, applyDirection);
 
             Result = new PatternTileResult