refactor: Move formatting utilities to CutList.Core.Formatting namespace

Consolidates ArchUnits, FormatHelper, and Fraction classes into a
dedicated Formatting namespace for better organization.

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

CutList.Core/Formatting/ArchUnits.cs

@@ -1,8 +1,7 @@
-using System;
 using System.Text;
 using System.Text.RegularExpressions;
 
-namespace CutList.Core
+namespace CutList.Core.Formatting
 {
     public static class ArchUnits
     {

CutList.Core/Formatting/FormatHelper.cs

@@ -1,6 +1,4 @@
-using System;
-
-namespace CutList.Core
+namespace CutList.Core.Formatting
 {
     /// <summary>
     /// Provides formatting utilities for displaying measurements and values.

CutList.Core/Formatting/Fraction.cs

@@ -1,9 +1,7 @@
-using System;
-using System.Linq;
 using System.Text;
 using System.Text.RegularExpressions;
 
-namespace CutList.Core
+namespace CutList.Core.Formatting
 {
     public static class Fraction
     {