feat(ExcelIO): add ColumnDefinition for fluent mapping

NEW/src/JdeScoping.ExcelIO/Mapping/ColumnDefinition.cs

@@ -0,0 +1,34 @@
+namespace JdeScoping.ExcelIO.Mapping;
+
+/// <summary>
+/// Defines how a property maps to an Excel column.
+/// </summary>
+public sealed class ColumnDefinition
+{
+    /// <summary>Property name for debugging/error messages.</summary>
+    public required string PropertyName { get; init; }
+
+    /// <summary>Compiled delegate to get property value from object.</summary>
+    public required Func<object, object?> ValueGetter { get; init; }
+
+    /// <summary>Property type for formatting decisions.</summary>
+    public required Type PropertyType { get; init; }
+
+    /// <summary>Column display order (lower = leftmost).</summary>
+    public int Order { get; set; }
+
+    /// <summary>Column header text.</summary>
+    public string HeaderText { get; set; } = string.Empty;
+
+    /// <summary>Excel number format string.</summary>
+    public string Format { get; set; } = "@";
+
+    /// <summary>Auto-size column width.</summary>
+    public bool AutoWidth { get; set; } = true;
+
+    /// <summary>Manual column width (when AutoWidth = false).</summary>
+    public double Width { get; set; }
+
+    /// <summary>Enable text wrapping.</summary>
+    public bool WrapText { get; set; }
+}