refactor(engine): align namespaces with directory layout under OpenNest.Engine

All 132 OpenNest.Engine source files now declare namespaces matching
their nested directories: Jobs/, Jobs/Placement/, Jobs/Adapters/,
Fill/, RectanglePacking/, CirclePacking/, and engine-root types moved
from 'OpenNest' to 'OpenNest.Engine'. RootNamespace updated accordingly.
Consumers (Api, Console, Mcp, Benchmark, Training, desktop app, tests)
gained the explicit usings the move requires; CLAUDE.md updated.

OpenNest.Engine/RectanglePacking/Bin.cs

@@ -2,7 +2,7 @@
 using System.Linq;
 using OpenNest.Geometry;
 
-namespace OpenNest.RectanglePacking
+namespace OpenNest.Engine.RectanglePacking
 {
     internal class Bin : Box
     {

OpenNest.Engine/RectanglePacking/BinConverter.cs

@@ -2,7 +2,7 @@ using System.Collections.Generic;
 using OpenNest.Geometry;
 using OpenNest.Math;
 
-namespace OpenNest.RectanglePacking
+namespace OpenNest.Engine.RectanglePacking
 {
     internal static class BinConverter
     {

OpenNest.Engine/RectanglePacking/FillBestFit.cs

@@ -1,7 +1,8 @@
 using System;
+using OpenNest.Engine.Fill;
 using OpenNest.Math;
 
-namespace OpenNest.RectanglePacking
+namespace OpenNest.Engine.RectanglePacking
 {
     internal class FillBestFit : FillEngine
     {

OpenNest.Engine/RectanglePacking/FillEngine.cs

@@ -1,7 +1,7 @@
 using System.Collections.Generic;
 using OpenNest.Geometry;
 
-namespace OpenNest.RectanglePacking
+namespace OpenNest.Engine.RectanglePacking
 {
     internal abstract class FillEngine
     {

OpenNest.Engine/RectanglePacking/FillNoRotation.cs

@@ -1,7 +1,7 @@
 using OpenNest.Geometry;
 using OpenNest.Math;
 
-namespace OpenNest.RectanglePacking
+namespace OpenNest.Engine.RectanglePacking
 {
     internal class FillNoRotation : FillEngine
     {

OpenNest.Engine/RectanglePacking/FillSameRotation.cs

@@ -1,6 +1,6 @@
 using OpenNest.Math;
 
-namespace OpenNest.RectanglePacking
+namespace OpenNest.Engine.RectanglePacking
 {
     internal class FillSameRotation : FillEngine
     {

OpenNest.Engine/RectanglePacking/FillSpiral.cs

@@ -1,7 +1,8 @@
 using OpenNest.Geometry;
+using OpenNest.Engine.Fill;
 using OpenNest.Math;
 
-namespace OpenNest.RectanglePacking
+namespace OpenNest.Engine.RectanglePacking
 {
     internal class FillSpiral : FillEngine
     {

OpenNest.Engine/RectanglePacking/Item.cs

@@ -2,7 +2,7 @@
 using OpenNest.Geometry;
 using OpenNest.Math;
 
-namespace OpenNest.RectanglePacking
+namespace OpenNest.Engine.RectanglePacking
 {
     internal class Item : Box
     {

OpenNest.Engine/RectanglePacking/PackBottomLeft.cs

@@ -3,7 +3,7 @@ using System.Linq;
 using OpenNest.Geometry;
 using OpenNest.Math;
 
-namespace OpenNest.RectanglePacking
+namespace OpenNest.Engine.RectanglePacking
 {
     internal class PackBottomLeft : PackEngine
     {

OpenNest.Engine/RectanglePacking/PackEngine.cs

@@ -1,6 +1,6 @@
 using System.Collections.Generic;
 
-namespace OpenNest.RectanglePacking
+namespace OpenNest.Engine.RectanglePacking
 {
     internal abstract class PackEngine
     {

OpenNest.Engine/RectanglePacking/PackFirstFitDecreasing.cs

@@ -2,7 +2,7 @@
 using System.Linq;
 using OpenNest.Geometry;
 
-namespace OpenNest.RectanglePacking
+namespace OpenNest.Engine.RectanglePacking
 {
     internal class FirstFitDecreasing : PackEngine
     {

OpenNest.Engine/RectanglePacking/RectFill.cs

@@ -1,6 +1,6 @@
 using OpenNest.Geometry;
 
-namespace OpenNest.RectanglePacking
+namespace OpenNest.Engine.RectanglePacking
 {
     internal static class RectFill
     {