Extract business logic into dedicated service layer

Create CutListService and DocumentService to separate business
logic from UI concerns, following the service layer pattern.

Changes:
- CutListService: Handles bin packing algorithm orchestration
  and data transformation from UI models to domain models
- DocumentService: Handles document persistence and validation
- Document: Removed Save/Load/Validate methods (now pure POCO)
- MainForm: Refactored to delegate to service classes instead
  of containing business logic directly

Benefits:
- Improved separation of concerns
- Business logic can now be unit tested independently
- Removed MessageBox dependencies from business logic layer
- MainForm is now focused on UI concerns only

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

CutList/Services/CutListService.cs

@@ -0,0 +1,74 @@
+using CutList.Models;
+using SawCut;
+using SawCut.Nesting;
+using System.Collections.Generic;
+
+namespace CutList.Services
+{
+    /// <summary>
+    /// Service class that handles the core business logic for cut list optimization.
+    /// Separates business logic from UI concerns.
+    /// </summary>
+    public class CutListService
+    {
+        /// <summary>
+        /// Runs the bin packing algorithm to optimize cut lists.
+        /// </summary>
+        /// <param name="parts">The parts to be nested</param>
+        /// <param name="stockBins">The available stock bins</param>
+        /// <param name="cuttingTool">The cutting tool to use (determines kerf/spacing)</param>
+        /// <returns>The packing result with optimized bins and unused items</returns>
+        public Result Pack(List<PartInputItem> parts, List<BinInputItem> stockBins, Tool cuttingTool)
+        {
+            var multiBins = ConvertToMultiBins(stockBins);
+            var binItems = ConvertToBinItems(parts);
+
+            var engine = new MultiBinEngine
+            {
+                Spacing = cuttingTool.Kerf,
+                Bins = multiBins
+            };
+
+            return engine.Pack(binItems);
+        }
+
+        private List<MultiBin> ConvertToMultiBins(List<BinInputItem> stockBins)
+        {
+            var multiBins = new List<MultiBin>();
+
+            foreach (var item in stockBins)
+            {
+                multiBins.Add(new MultiBin
+                {
+                    Length = item.Length.Value,
+                    Quantity = item.Quantity,
+                    Priority = item.Priority
+                });
+            }
+
+            return multiBins;
+        }
+
+        private List<BinItem> ConvertToBinItems(List<PartInputItem> parts)
+        {
+            var binItems = new List<BinItem>();
+
+            foreach (var part in parts)
+            {
+                if (part.Length == null || part.Length == 0)
+                    continue;
+
+                for (int i = 0; i < part.Quantity; i++)
+                {
+                    binItems.Add(new BinItem
+                    {
+                        Name = part.Name,
+                        Length = part.Length.Value
+                    });
+                }
+            }
+
+            return binItems;
+        }
+    }
+}

CutList/Services/DocumentService.cs

@@ -0,0 +1,64 @@
+using CutList.Forms;
+using Newtonsoft.Json;
+using System;
+using System.IO;
+
+namespace CutList.Services
+{
+    /// <summary>
+    /// Service class that handles document persistence operations.
+    /// Separates file I/O logic from UI concerns.
+    /// </summary>
+    public class DocumentService
+    {
+        /// <summary>
+        /// Saves a document to the specified file path.
+        /// </summary>
+        /// <param name="document">The document to save</param>
+        /// <param name="filePath">The file path to save to</param>
+        /// <exception cref="IOException">Thrown when file cannot be saved</exception>
+        public void Save(Document document, string filePath)
+        {
+            var json = JsonConvert.SerializeObject(document, Formatting.Indented);
+            File.WriteAllText(filePath, json);
+        }
+
+        /// <summary>
+        /// Loads a document from the specified file path.
+        /// </summary>
+        /// <param name="filePath">The file path to load from</param>
+        /// <returns>The loaded document</returns>
+        /// <exception cref="IOException">Thrown when file cannot be read</exception>
+        /// <exception cref="JsonException">Thrown when file contains invalid JSON</exception>
+        public Document Load(string filePath)
+        {
+            var json = File.ReadAllText(filePath);
+            var document = JsonConvert.DeserializeObject<Document>(json);
+            return document;
+        }
+
+        /// <summary>
+        /// Validates that a document has the minimum required data.
+        /// </summary>
+        /// <param name="document">The document to validate</param>
+        /// <param name="validationMessage">Output parameter containing validation error message</param>
+        /// <returns>True if document is valid, false otherwise</returns>
+        public bool Validate(Document document, out string validationMessage)
+        {
+            if (document.PartsToNest == null || document.PartsToNest.Count == 0)
+            {
+                validationMessage = "No parts to nest.";
+                return false;
+            }
+
+            if (document.StockBins == null || document.StockBins.Count == 0)
+            {
+                validationMessage = "No stock bins available.";
+                return false;
+            }
+
+            validationMessage = string.Empty;
+            return true;
+        }
+    }
+}