feat: smart strategy skipping, pack rotation, and dual-sort packing

- Skip ExtentsFillStrategy for rectangle/circle parts
- Skip PairsFillStrategy for circle parts
- PackBottomLeft now tries rotated orientation when items don't fit
- PackBottomLeft tries both area-descending and length-descending sort
  orders, keeping whichever places more parts (tighter bbox on tie)
- Add user constraint override tests for AngleCandidateBuilder

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

OpenNest.Engine/Strategies/ExtentsFillStrategy.cs

@@ -12,6 +12,8 @@ namespace OpenNest.Engine.Strategies
 
         public List<Part> Fill(FillContext context)
         {
+            if (context.PartType == PartType.Rectangle || context.PartType == PartType.Circle)
+                return null;
             var filler = new FillExtents(context.WorkArea, context.Plate.PartSpacing);
 
             var bestRotation = context.SharedState.TryGetValue("BestRotation", out var rot)