feat: fast-path fill and dual-axis shrink for low quantities

For qty 1-2, skip the full 6-strategy pipeline: place a single part
or a best-fit pair directly. For larger low quantities, shrink the
work area in both dimensions (sqrt scaling with 2x margin) before
running strategies, with fallback to full area if insufficient.

- Add TryFillSmallQuantity fast path (qty=1 single, qty=2 best-fit pair)
- Add ShrinkWorkArea with proportional dual-axis reduction
- Extract RunFillPipeline helper from Fill()
- Make ShrinkFiller.EstimateStartBox internal with margin parameter
- Add MaxQuantity to FillContext for strategy-level access

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

OpenNest.Engine/Strategies/FillContext.cs

@@ -16,6 +16,7 @@ namespace OpenNest.Engine.Strategies
         public CancellationToken Token { get; init; }
         public IProgress<NestProgress> Progress { get; init; }
         public FillPolicy Policy { get; init; }
+        public int MaxQuantity { get; init; }
         public PartType PartType { get; set; }
 
         public List<Part> CurrentBest { get; set; }