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/PairsFillStrategy.cs

@@ -20,6 +20,9 @@ namespace OpenNest.Engine.Strategies
             if (active.Value)
                 return null;
 
+            if (context.PartType == PartType.Circle)
+                return null;
+
             active.Value = true;
             try
             {