Rename SawCut library to CutList.Core

Rename the core library project from SawCut to CutList.Core for consistent
branding across the solution. This includes:
- Rename project folder and .csproj file
- Update namespace from SawCut to CutList.Core
- Update all using statements and project references

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

CutList.Core/BinItem.cs

@@ -0,0 +1,97 @@
+using System;
+
+namespace CutList.Core
+{
+    /// <summary>
+    /// Represents an item to be placed in a bin.
+    /// Enforces business rules for valid items.
+    /// </summary>
+    public class BinItem
+    {
+        private string _name;
+        private double _length;
+
+        public BinItem(string name, double length)
+        {
+            if (string.IsNullOrWhiteSpace(name))
+                throw new ArgumentException("Item name cannot be empty", nameof(name));
+
+            if (length <= 0)
+                throw new ArgumentException("Item length must be greater than zero", nameof(length));
+
+            _name = name;
+            _length = length;
+        }
+
+        /// <summary>
+        /// Parameterless constructor for serialization only.
+        /// Use the parameterized constructor for creating valid instances.
+        /// </summary>
+        public BinItem()
+        {
+        }
+
+        public string Name
+        {
+            get => _name;
+            set
+            {
+                if (string.IsNullOrWhiteSpace(value))
+                    throw new ArgumentException("Item name cannot be empty", nameof(value));
+                _name = value;
+            }
+        }
+
+        public double Length
+        {
+            get => _length;
+            set
+            {
+                if (value <= 0)
+                    throw new ArgumentException("Item length must be greater than zero", nameof(value));
+                _length = value;
+            }
+        }
+
+        /// <summary>
+        /// Checks if this item can fit in the given available length.
+        /// </summary>
+        public bool CanFitIn(double availableLength)
+        {
+            return Length <= availableLength;
+        }
+
+        /// <summary>
+        /// Checks if this item can fit in the given available length with spacing.
+        /// </summary>
+        public bool CanFitInWithSpacing(double availableLength, double spacing)
+        {
+            return Length + spacing <= availableLength;
+        }
+
+        public override string ToString()
+        {
+            return $"{Name} ({Length}\")";
+        }
+
+        public override bool Equals(object? obj)
+        {
+            if (obj is BinItem other)
+            {
+                return Name == other.Name && Length.IsEqualTo(other.Length);
+            }
+            return false;
+        }
+
+        public override int GetHashCode()
+        {
+            unchecked
+            {
+                int hash = 17;
+                hash = hash * 23 + (Name?.GetHashCode() ?? 0);
+                hash = hash * 23 + Length.GetHashCode();
+                return hash;
+            }
+        }
+    }
+}