Initial commit: JDE Scoping Tool migration project

Set up repository with legacy .NET Framework 4.8 source (OLD/),
new .NET 10 Blazor solution (NEW/), OpenSpec specifications,
documentation, and project configuration.

NEW/src/JdeScoping.Core/Interfaces/IExcelTemplateService.cs

@@ -0,0 +1,24 @@
+namespace JdeScoping.Core.Interfaces;
+
+/// <summary>
+/// Service for generating Excel template files for data entry.
+/// </summary>
+public interface IExcelTemplateService
+{
+    /// <summary>
+    /// Generates an Excel file with a single column of data.
+    /// </summary>
+    /// <typeparam name="T">Type of data items.</typeparam>
+    /// <param name="data">Data items to write.</param>
+    /// <param name="headerText">Header text for the column.</param>
+    /// <returns>Excel file as byte array.</returns>
+    byte[] GenerateSingleColumn<T>(IEnumerable<T> data, string headerText);
+
+    /// <summary>
+    /// Generates an Excel file with multiple columns of data.
+    /// </summary>
+    /// <param name="data">2D array of data (rows x columns).</param>
+    /// <param name="headers">Header text for each column.</param>
+    /// <returns>Excel file as byte array.</returns>
+    byte[] GenerateMultiColumn(object?[][] data, string[] headers);
+}