feat(engine): add LinearFillStrategy adapter

Wraps FillLinear in an IFillStrategy, sweeping all AngleCandidates
from SharedState (falling back to 0° and 90°) in both directions and
recording AngleResults for UI inspection.

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

OpenNest.Engine/Strategies/LinearFillStrategy.cs

@@ -0,0 +1,75 @@
+using System.Collections.Generic;
+using OpenNest.Math;
+
+namespace OpenNest
+{
+    public class LinearFillStrategy : IFillStrategy
+    {
+        public string Name => "Linear";
+        public NestPhase Phase => NestPhase.Linear;
+        public int Order => 400;
+
+        public List<Part> Fill(FillContext context)
+        {
+            var angles = context.SharedState.TryGetValue("AngleCandidates", out var cached)
+                ? (List<double>)cached
+                : new List<double> { 0, Angle.HalfPI };
+
+            var workArea = context.WorkArea;
+            List<Part> best = null;
+            var bestScore = default(FillScore);
+
+            for (var ai = 0; ai < angles.Count; ai++)
+            {
+                context.Token.ThrowIfCancellationRequested();
+
+                var angle = angles[ai];
+                var engine = new FillLinear(workArea, context.Plate.PartSpacing);
+                var h = engine.Fill(context.Item.Drawing, angle, NestDirection.Horizontal);
+                var v = engine.Fill(context.Item.Drawing, angle, NestDirection.Vertical);
+
+                var angleDeg = Angle.ToDegrees(angle);
+
+                if (h != null && h.Count > 0)
+                {
+                    var scoreH = FillScore.Compute(h, workArea);
+                    context.AngleResults.Add(new AngleResult
+                    {
+                        AngleDeg = angleDeg,
+                        Direction = NestDirection.Horizontal,
+                        PartCount = h.Count
+                    });
+
+                    if (best == null || scoreH > bestScore)
+                    {
+                        best = h;
+                        bestScore = scoreH;
+                    }
+                }
+
+                if (v != null && v.Count > 0)
+                {
+                    var scoreV = FillScore.Compute(v, workArea);
+                    context.AngleResults.Add(new AngleResult
+                    {
+                        AngleDeg = angleDeg,
+                        Direction = NestDirection.Vertical,
+                        PartCount = v.Count
+                    });
+
+                    if (best == null || scoreV > bestScore)
+                    {
+                        best = v;
+                        bestScore = scoreV;
+                    }
+                }
+
+                NestEngineBase.ReportProgress(context.Progress, NestPhase.Linear,
+                    context.PlateNumber, best, workArea,
+                    $"Linear: {ai + 1}/{angles.Count} angles, {angleDeg:F0}° best = {bestScore.Count} parts");
+            }
+
+            return best ?? new List<Part>();
+        }
+    }
+}