fix(geometry): correct overlap detection for real-world CNC shapes

Part.Intersects has been silently non-functional everywhere it's used.
Shape.ToPolygon()/ToPolygonWithTolerance() never called UpdateBounds(),
so every freshly-built polygon kept Entity's constructor-default
zero-size bounding box regardless of its actual vertices. Collision.Check's
first step is a bounding-box pre-filter, and a zero-size box can never
overlap anything, so it always short-circuited to "no overlap" no matter
what the real geometry looked like.

That masked a second bug: Part.Intersects built its polygons via the
unconditional 1000-segments-per-arc ToPolygon() default instead of an
adaptive tolerance. For parts with several small fillets/holes this
produced tens of thousands of vertices, making the now-correct bbox
check fall through into a triangulation/clip step too slow to return
in practice. Switched to ToPolygonWithTolerance at a named tolerance
matching PartGeometry's existing convention.

PairEvaluator's own Keep/overlap check had a third, independent bug:
it used Shape.Intersects (edge-crossing detection only) at a coarse
0.01 chord tolerance, which misses containment-style overlaps and can
polygonize rounded corners coarsely enough to hide a genuine sliver
overlap. Switched to Collision.HasOverlap (full polygon clip, handles
containment) at a tighter dedicated tolerance.

Verified against a real nest file: PairFiller was tiling a BestFit
pair that PairEvaluator had incorrectly marked Keep=true, producing
visibly overlapping parts on the plate that no downstream overlap
check ever caught.

Known follow-up: OpenNest.Tests.BestFit.BestFitOverlapTests.KeptPairs_NoOverlap
still fails on 3/1082 synthetic candidates that overlap by a sub-0.001
sliver right at a rounded-corner tangent point — a separate, much
smaller precision edge case in PairEvaluator's raw (pre-transform)
coordinate frame, not a regression from this change.

OpenNest.Engine/BestFit/PairEvaluator.cs

@@ -13,6 +13,15 @@ namespace OpenNest.Engine.BestFit
     {
         private const double ChordTolerance = 0.01;
 
+        /// <summary>
+        /// Tighter chord tolerance for the overlap check only. Rounded-corner arcs
+        /// polygonized at the coarser <see cref="ChordTolerance"/> can "cut the corner"
+        /// enough to hide a genuine but tiny sliver overlap between two candidates —
+        /// this needs to match the precision Part.Intersects uses elsewhere so BestFit's
+        /// Keep decision agrees with the same overlap check callers rely on downstream.
+        /// </summary>
+        private const double OverlapChordTolerance = 0.001;
+
         public List<BestFitResult> EvaluateAll(List<PairCandidate> candidates)
         {
             if (candidates.Count == 0)
@@ -49,10 +58,18 @@ namespace OpenNest.Engine.BestFit
             part2.Location = candidate.Part2Offset;
             part2.UpdateBounds();
 
-            // Overlap check — perimeter vs perimeter
+            // Overlap check — perimeter vs perimeter. Uses Collision.HasOverlap (full polygon
+            // clip) rather than Shape.Intersects (edge-crossing only), which misses containment-
+            // style overlaps where one perimeter's boundary never crosses the other's.
             var shape1 = GetPerimeterShape(part1);
             var shape2 = GetPerimeterShape(part2);
-            var overlaps = shape1 != null && shape2 != null && shape1.Intersects(shape2, out _);
+            var overlaps =
+                shape1 != null
+                && shape2 != null
+                && Collision.HasOverlap(
+                    shape1.ToPolygonWithTolerance(OverlapChordTolerance),
+                    shape2.ToPolygonWithTolerance(OverlapChordTolerance)
+                );
 
             // Convex hull vertices from perimeter polygons only
             var allPoints = GetPartVertices(part1);