feat(engine): add IFillStrategy interface and FillContext

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

OpenNest.Engine/Strategies/FillContext.cs

@@ -0,0 +1,25 @@
+using System;
+using System.Collections.Generic;
+using System.Threading;
+using OpenNest.Geometry;
+
+namespace OpenNest
+{
+    public class FillContext
+    {
+        public NestItem Item { get; init; }
+        public Box WorkArea { get; init; }
+        public Plate Plate { get; init; }
+        public int PlateNumber { get; init; }
+        public CancellationToken Token { get; init; }
+        public IProgress<NestProgress> Progress { get; init; }
+
+        public List<Part> CurrentBest { get; set; }
+        public FillScore CurrentBestScore { get; set; }
+        public NestPhase WinnerPhase { get; set; }
+        public List<PhaseResult> PhaseResults { get; } = new();
+        public List<AngleResult> AngleResults { get; } = new();
+
+        public Dictionary<string, object> SharedState { get; } = new();
+    }
+}

OpenNest.Engine/Strategies/IFillStrategy.cs

@@ -0,0 +1,12 @@
+using System.Collections.Generic;
+
+namespace OpenNest
+{
+    public interface IFillStrategy
+    {
+        string Name { get; }
+        NestPhase Phase { get; }
+        int Order { get; }
+        List<Part> Fill(FillContext context);
+    }
+}