Add validation and business logic to domain models

Enhance Tool, BinItem, and MultiBin classes with:
- Input validation in constructors and property setters
- Guard clauses for invalid states (negative values, empty names)
- Business logic methods (CanFitItem, CalculateWaste, etc.)
- Proper encapsulation with backing fields
- Comprehensive XML documentation
- Override Equals/GetHashCode for value semantics
- Better ToString() implementations

These changes enforce business rules at the domain level and prevent
invalid states from being created.

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

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

CutList/Tool.cs

@@ -1,16 +1,109 @@
-namespace CutList
+using System;
+
+namespace CutList
 {
+    /// <summary>
+    /// Represents a cutting tool with its kerf (blade width).
+    /// Enforces business rules for valid tools.
+    /// </summary>
     public class Tool
     {
-        public string Name { get; set; }
+        private string _name;
+        private double _kerf;
 
-        public double Kerf { get; set; }
+        public Tool(string name, double kerf, bool allowUserToChange = false)
+        {
+            if (string.IsNullOrWhiteSpace(name))
+                throw new ArgumentException("Tool name cannot be empty", nameof(name));
+
+            if (kerf < 0)
+                throw new ArgumentException("Kerf cannot be negative", nameof(kerf));
+
+            _name = name;
+            _kerf = kerf;
+            AllowUserToChange = allowUserToChange;
+        }
+
+        /// <summary>
+        /// Parameterless constructor for serialization only.
+        /// Use the parameterized constructor for creating valid instances.
+        /// </summary>
+        public Tool()
+        {
+        }
+
+        public string Name
+        {
+            get => _name;
+            set
+            {
+                if (string.IsNullOrWhiteSpace(value))
+                    throw new ArgumentException("Tool name cannot be empty", nameof(value));
+                _name = value;
+            }
+        }
+
+        public double Kerf
+        {
+            get => _kerf;
+            set
+            {
+                if (value < 0)
+                    throw new ArgumentException("Kerf cannot be negative", nameof(value));
+                _kerf = value;
+            }
+        }
 
         public bool AllowUserToChange { get; set; }
 
+        /// <summary>
+        /// Calculates the total material wasted for a given number of cuts.
+        /// </summary>
+        public double CalculateTotalWaste(int numberOfCuts)
+        {
+            if (numberOfCuts < 0)
+                throw new ArgumentException("Number of cuts cannot be negative", nameof(numberOfCuts));
+
+            return Kerf * numberOfCuts;
+        }
+
+        /// <summary>
+        /// Checks if this tool is compatible with the given material thickness.
+        /// Some tools may have minimum/maximum thickness requirements.
+        /// </summary>
+        public bool IsCompatibleWithThickness(double thickness)
+        {
+            // For now, all tools are compatible with any thickness
+            // This can be extended based on specific tool requirements
+            return thickness > 0;
+        }
+
         public override string ToString()
         {
             return Name;
         }
+
+        public override bool Equals(object obj)
+        {
+            if (obj is Tool other)
+            {
+                return Name == other.Name &&
+                       Math.Abs(Kerf - other.Kerf) < 0.0001 &&
+                       AllowUserToChange == other.AllowUserToChange;
+            }
+            return false;
+        }
+
+        public override int GetHashCode()
+        {
+            unchecked
+            {
+                int hash = 17;
+                hash = hash * 23 + (Name?.GetHashCode() ?? 0);
+                hash = hash * 23 + Kerf.GetHashCode();
+                hash = hash * 23 + AllowUserToChange.GetHashCode();
+                return hash;
+            }
+        }
     }
 }