fix(geometry): end PolyLabel's starting grid where steps stop advancing

Far from the origin a grid step smaller than the coordinate's
precision leaves x (or y) unchanged: at 1e16, 1e16 + 1 == 1e16. The
starting grid loop then never ended and allocated cells without limit,
even with the new maxCells budget (a hang dump reached 4.3 GB). Stop a
row or column once its step no longer advances.

Grids that advance are untouched, so the bit-identical differential
against the former sorted-list implementation still passes. Found by
the independent quality review of the Clipper2 overlap branch.

OpenNest.Core/Geometry/PolyLabel.cs

@@ -73,8 +73,18 @@ namespace OpenNest.Geometry
             var initial = new List<Cell>();
 
             for (var x = minX; x < maxX; x += cellSize)
+            {
                 for (var y = minY; y < maxY; y += cellSize)
+                {
                     initial.Add(new Cell(x + halfCell, y + halfCell, halfCell, outer, holes));
+                    // Far from the origin a step smaller than the coordinate's precision
+                    // no longer advances; the grid would never end.
+                    if (y + cellSize == y)
+                        break;
+                }
+                if (x + cellSize == x)
+                    break;
+            }
 
             initial.Sort((a, b) => b.MaxDist.CompareTo(a.MaxDist));
 

OpenNest.Tests/Geometry/PolyLabelDifferentialTests.cs

@@ -68,6 +68,21 @@ public class PolyLabelDifferentialTests
         Assert.True(Same(PolyLabel.Find(shape), PolyLabel.Find(shape, null, 0.5, int.MaxValue)));
     }
 
+    // Quality review: at 1e16 a 1-unit grid step no longer changes x (1e16 + 1 == 1e16), and
+    // the starting grid loop never ended, allocating cells without limit even with a budget.
+    [Fact]
+    public async Task Find_FinishesWhenGridStepsStopAdvancingFarFromTheOrigin()
+    {
+        var far = Rectangle(1e16, 0, 8, 1);
+
+        var search = Task.Run(() => (PolyLabel.Find(far, null, 0.01, 4096), PolyLabel.Find(far, null, 0.01)));
+
+        Assert.Same(search, await Task.WhenAny(search, Task.Delay(TimeSpan.FromSeconds(20))));
+        var (bounded, unbounded) = await search;
+        Assert.InRange(bounded.Y, 0, 1);
+        Assert.InRange(unbounded.Y, 0, 1);
+    }
+
     private static (Polygon outer, List<Polygon> holes) RandomShape(Random random)
     {
         var scale = System.Math.Pow(10, random.NextDouble() * 4 - 1);