feat: geometry-aware convergence, both-axis search, remnant engine, fill cache

- Convergence loop now uses FillLinear internally to measure actual
  waste with geometry-aware spacing instead of bounding-box arithmetic
- Each candidate pair is tried in both Row and Column orientations to
  find the shortest perpendicular dimension (more complete stripes)
- CompleteStripesOnly flag drops partial stripes; remnant strip is
  filled by a full engine run (injected via CreateRemnantEngine)
- ConvergeStripeAngleShrink tries N+1 narrower pairs as alternative
- FillResultCache avoids redundant engine runs on same-sized remnants
- CLAUDE.md: note to not commit specs/plans

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

OpenNest.Engine/Strategies/RowFillStrategy.cs

@@ -11,7 +11,7 @@ public class RowFillStrategy : IFillStrategy
 
     public List<Part> Fill(FillContext context)
     {
-        var filler = new StripeFiller(context, NestDirection.Horizontal);
+        var filler = new StripeFiller(context, NestDirection.Horizontal) { CompleteStripesOnly = true };
         return filler.Fill();
     }
 }