refactor(engine): move fill and strategy code to dedicated namespaces

Move fill algorithms to OpenNest.Engine.Fill namespace:
FillLinear, FillExtents, PairFiller, ShrinkFiller, Compactor,
RemnantFiller, RemnantFinder, FillScore, Pattern, PatternTiler,
PartBoundary, RotationAnalysis, AngleCandidateBuilder, and
AccumulatingProgress.

Move strategy layer to OpenNest.Engine.Strategies namespace:
IFillStrategy, FillContext, FillStrategyRegistry, FillHelpers,
and all built-in strategy implementations.

Add using directives to all consuming files across Engine, UI,
MCP, and Tests projects.

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

OpenNest.Engine/Fill/AccumulatingProgress.cs

@@ -1,7 +1,7 @@
 using System;
 using System.Collections.Generic;
 
-namespace OpenNest
+namespace OpenNest.Engine.Fill
 {
     /// <summary>
     /// Wraps an IProgress to prepend previously placed parts to each report,

OpenNest.Engine/Fill/AngleCandidateBuilder.cs

@@ -1,11 +1,11 @@
-using System.Collections.Generic;
-using System.Diagnostics;
-using System.Linq;
 using OpenNest.Engine.ML;
 using OpenNest.Geometry;
 using OpenNest.Math;
+using System.Collections.Generic;
+using System.Diagnostics;
+using System.Linq;
 
-namespace OpenNest
+namespace OpenNest.Engine.Fill
 {
     /// <summary>
     /// Builds candidate rotation angles for single-item fill. Encapsulates the

OpenNest.Engine/Fill/Compactor.cs

@@ -1,9 +1,8 @@
-using System;
+using OpenNest.Geometry;
 using System.Collections.Generic;
 using System.Linq;
-using OpenNest.Geometry;
 
-namespace OpenNest
+namespace OpenNest.Engine.Fill
 {
     /// <summary>
     /// Pushes a group of parts left and down to close gaps after placement.

OpenNest.Engine/Fill/FillExtents.cs

@@ -1,11 +1,11 @@
+using OpenNest.Geometry;
+using OpenNest.Math;
 using System;
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.Threading;
-using OpenNest.Geometry;
-using OpenNest.Math;
 
-namespace OpenNest
+namespace OpenNest.Engine.Fill
 {
     public class FillExtents
     {

OpenNest.Engine/Fill/FillLinear.cs

@@ -1,9 +1,9 @@
-using System.Collections.Generic;
-using System.Threading.Tasks;
 using OpenNest.Geometry;
 using OpenNest.Math;
+using System.Collections.Generic;
+using System.Threading.Tasks;
 
-namespace OpenNest
+namespace OpenNest.Engine.Fill
 {
     public class FillLinear
     {
@@ -16,7 +16,7 @@ namespace OpenNest
         public Box WorkArea { get; }
 
         public double PartSpacing { get; }
-        
+
         public double HalfSpacing => PartSpacing / 2;
 
         /// <summary>

OpenNest.Engine/Fill/FillScore.cs

@@ -1,7 +1,7 @@
-using System.Collections.Generic;
 using OpenNest.Geometry;
+using System.Collections.Generic;
 
-namespace OpenNest
+namespace OpenNest.Engine.Fill
 {
     public readonly struct FillScore : System.IComparable<FillScore>
     {

OpenNest.Engine/Fill/PairFiller.cs

@@ -1,13 +1,14 @@
+using OpenNest.Engine.BestFit;
+using OpenNest.Engine.Strategies;
+using OpenNest.Geometry;
+using OpenNest.Math;
 using System;
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.Linq;
 using System.Threading;
-using OpenNest.Engine.BestFit;
-using OpenNest.Geometry;
-using OpenNest.Math;
 
-namespace OpenNest
+namespace OpenNest.Engine.Fill
 {
     /// <summary>
     /// Fills a work area using interlocking part pairs from BestFitCache.
@@ -52,11 +53,11 @@ namespace OpenNest
                     var engine = new FillLinear(workArea, partSpacing);
 
                     // Let the remainder strip try pair-based filling too.
-                    var p0 = DefaultNestEngine.BuildRotatedPattern(pairParts, 0);
-                    var p90 = DefaultNestEngine.BuildRotatedPattern(pairParts, Angle.HalfPI);
+                    var p0 = FillHelpers.BuildRotatedPattern(pairParts, 0);
+                    var p90 = FillHelpers.BuildRotatedPattern(pairParts, Angle.HalfPI);
                     engine.RemainderPatterns = new List<Pattern> { p0, p90 };
 
-                    var filled = DefaultNestEngine.FillPattern(engine, pairParts, angles, workArea);
+                    var filled = FillHelpers.FillPattern(engine, pairParts, angles, workArea);
 
                     if (filled != null && filled.Count > 0)
                     {

OpenNest.Engine/Fill/PartBoundary.cs

@@ -1,9 +1,9 @@
-using System.Collections.Generic;
-using System.Linq;
 using OpenNest.Converters;
 using OpenNest.Geometry;
+using System.Collections.Generic;
+using System.Linq;
 
-namespace OpenNest
+namespace OpenNest.Engine.Fill
 {
     /// <summary>
     /// Pre-computed offset boundary polygons for a part's geometry.
@@ -87,9 +87,9 @@ namespace OpenNest
                     var edge = (verts[i - 1], verts[i]);
 
                     if (-sign * dy > 0) left.Add(edge);
-                    if ( sign * dy > 0) right.Add(edge);
+                    if (sign * dy > 0) right.Add(edge);
                     if (-sign * dx > 0) up.Add(edge);
-                    if ( sign * dx > 0) down.Add(edge);
+                    if (sign * dx > 0) down.Add(edge);
                 }
             }
 
@@ -145,11 +145,11 @@ namespace OpenNest
         {
             switch (direction)
             {
-                case PushDirection.Left:  return _leftEdges;
+                case PushDirection.Left: return _leftEdges;
                 case PushDirection.Right: return _rightEdges;
-                case PushDirection.Up:    return _upEdges;
-                case PushDirection.Down:  return _downEdges;
-                default:                  return _leftEdges;
+                case PushDirection.Up: return _upEdges;
+                case PushDirection.Down: return _downEdges;
+                default: return _leftEdges;
             }
         }
 

OpenNest.Engine/Fill/Pattern.cs

@@ -1,7 +1,7 @@
-using System.Collections.Generic;
 using OpenNest.Geometry;
+using System.Collections.Generic;
 
-namespace OpenNest
+namespace OpenNest.Engine.Fill
 {
     public class Pattern
     {

OpenNest.Engine/Fill/PatternTiler.cs

@@ -1,7 +1,7 @@
-using System.Collections.Generic;
 using OpenNest.Geometry;
+using System.Collections.Generic;
 
-namespace OpenNest.Engine
+namespace OpenNest.Engine.Fill
 {
     public static class PatternTiler
     {

OpenNest.Engine/Fill/RemnantFiller.cs

@@ -1,9 +1,9 @@
+using OpenNest.Geometry;
 using System;
 using System.Collections.Generic;
 using System.Threading;
-using OpenNest.Geometry;
 
-namespace OpenNest
+namespace OpenNest.Engine.Fill
 {
     /// <summary>
     /// Iteratively fills remnant boxes with items using a RemnantFinder.

OpenNest.Engine/Fill/RemnantFinder.cs

@@ -1,9 +1,8 @@
-using System;
+using OpenNest.Geometry;
 using System.Collections.Generic;
 using System.Linq;
-using OpenNest.Geometry;
 
-namespace OpenNest
+namespace OpenNest.Engine.Fill
 {
     /// <summary>
     /// A remnant box with a priority tier.

OpenNest.Engine/Fill/RotationAnalysis.cs

@@ -1,10 +1,10 @@
-using System.Collections.Generic;
-using System.Linq;
 using OpenNest.Converters;
 using OpenNest.Geometry;
 using OpenNest.Math;
+using System.Collections.Generic;
+using System.Linq;
 
-namespace OpenNest
+namespace OpenNest.Engine.Fill
 {
     internal static class RotationAnalysis
     {

OpenNest.Engine/Fill/ShrinkFiller.cs

@@ -1,10 +1,10 @@
+using OpenNest.Geometry;
 using System;
 using System.Collections.Generic;
 using System.Linq;
 using System.Threading;
-using OpenNest.Geometry;
 
-namespace OpenNest
+namespace OpenNest.Engine.Fill
 {
     public enum ShrinkAxis { Width, Height }