fix(engine): prevent FillExtents overlap and add strategy filter API

FillExtents vertical copy distance was not clamped, allowing rows to be
placed overlapping each other when slide calculations returned large
values. Clamp to pairHeight + partSpacing minimum, matching FillLinear.

Also add FillStrategyRegistry.SetEnabled() to restrict which strategies
run — useful for isolating individual strategies during troubleshooting.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

OpenNest.Engine/Strategies/FillStrategyRegistry.cs

@@ -11,6 +11,7 @@ namespace OpenNest.Engine.Strategies
     {
         private static readonly List<IFillStrategy> strategies = new();
         private static List<IFillStrategy> sorted;
+        private static HashSet<string> enabledFilter;
 
         static FillStrategyRegistry()
         {
@@ -18,7 +19,21 @@ namespace OpenNest.Engine.Strategies
         }
 
         public static IReadOnlyList<IFillStrategy> Strategies =>
-            sorted ??= strategies.OrderBy(s => s.Order).ToList();
+            sorted ??= (enabledFilter != null
+                ? strategies.Where(s => enabledFilter.Contains(s.Name)).OrderBy(s => s.Order).ToList()
+                : strategies.OrderBy(s => s.Order).ToList());
+
+        /// <summary>
+        /// Restricts the active strategies to only those whose names are listed.
+        /// Pass null to restore all strategies.
+        /// </summary>
+        public static void SetEnabled(params string[] names)
+        {
+            enabledFilter = names != null && names.Length > 0
+                ? new HashSet<string>(names, StringComparer.OrdinalIgnoreCase)
+                : null;
+            sorted = null;
+        }
 
         public static void LoadFrom(Assembly assembly)
         {