perf: parallelize PairFiller candidates and add GridDedup

- Evaluate pair candidates in parallel batches instead of sequentially
- Add GridDedup to skip duplicate pattern/direction/workArea combos
  across PairFiller and StripeFiller strategies
- Replace crude 30% remnant area estimate with L-shaped geometry
  calculation using actual grid extents and max utilization
- Move FillStrategyRegistry.SetEnabled to outer evaluation loop
  to avoid repeated enable/disable per remnant fill

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

OpenNest.Engine/Strategies/PairsFillStrategy.cs

@@ -12,7 +12,8 @@ namespace OpenNest.Engine.Strategies
         public List<Part> Fill(FillContext context)
         {
             var comparer = context.Policy?.Comparer;
-            var filler = new PairFiller(context.Plate, comparer);
+            var dedup = GridDedup.GetOrCreate(context.SharedState);
+            var filler = new PairFiller(context.Plate, comparer, dedup);
             var result = filler.Fill(context.Item, context.WorkArea,
                 context.PlateNumber, context.Token, context.Progress);