perf(fill): reuse offset geometry for translated copies

FillLinear re-prepared offset perimeter geometry (ConvertProgram ->
ShapeProfile -> OffsetOutward) for every part it measured, although
tiled copies share one Program and differ only by Location. A CPU
profile of a 169-part Default job put 62% of wall time there.

Prepare each distinct Program (reference identity) once per public
Fill/FillRow call in local frame, then clone and translate for each
location. The cache is created per call and passed down privately
because FillHelpers.FillPattern calls Fill concurrently on one
instance. PartGeometry gains a local-frame Program overload that the
Part overload now delegates to.

Evaluation order, lazy preparation, fallbacks and tiling are
unchanged. Differential tests against a frozen copy of the previous
FillLinear check bitwise equality, including concurrent calls; Debug
work tests pin preparation counts. With the thread pool capped at one
worker, before/after whole-job layouts are byte-identical. The
Default corpus job median drops from 40,715 to 18,810 ms.

OpenNest.Core/PartGeometry.cs

@@ -52,9 +52,21 @@ namespace OpenNest
         /// without tessellation, which keeps arc-heavy parts fast in directional-distance loops.
         /// </summary>
         public static List<Entity> GetOffsetPerimeterEntities(Part part, double spacing)
+        {
+            var entities = GetOffsetPerimeterEntities(part.Program, spacing);
+            foreach (var entity in entities)
+                entity.Offset(part.Location);
+
+            return entities;
+        }
+
+        /// <summary>
+        /// Prepares a fresh offset perimeter in the program's local frame, without translation.
+        /// </summary>
+        public static List<Entity> GetOffsetPerimeterEntities(CNC.Program program, double spacing)
         {
             PerfCounters.CountOffsetPerimeterEntities();
-            var geoEntities = ConvertProgram.ToGeometry(part.Program);
+            var geoEntities = ConvertProgram.ToGeometry(program);
             var profile = new ShapeProfile(
                 geoEntities.Where(e => SpecialLayers.IsMaterial(e.Layer)).ToList()
             );
@@ -63,11 +75,6 @@ namespace OpenNest
             if (offsetShape == null)
                 return new List<Entity>();
 
-            // Offset the shape's entities to the part's location.
-            // OffsetOutward creates a new Shape, so mutating is safe.
-            foreach (var entity in offsetShape.Entities)
-                entity.Offset(part.Location);
-
             return offsetShape.Entities;
         }