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.DataSync/Contracts/IDataFetcher.cs

@@ -0,0 +1,23 @@
+namespace JdeScoping.DataSync.Contracts;
+
+/// <summary>
+/// Interface for fetching data from source systems (JDE/CMS).
+/// </summary>
+/// <typeparam name="TEntity">The entity type being fetched.</typeparam>
+public interface IDataFetcher<TEntity> where TEntity : class
+{
+    /// <summary>
+    /// Fetches entities from source system as an async stream.
+    /// </summary>
+    /// <param name="minimumDt">For incremental fetches, only return records modified after this time. Null for full fetch.</param>
+    /// <param name="cancellationToken">Cancellation token for graceful shutdown.</param>
+    /// <returns>Async enumerable of entities, streamed from source.</returns>
+    IAsyncEnumerable<TEntity> FetchAsync(
+        DateTime? minimumDt,
+        CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Gets the entity type name for logging purposes.
+    /// </summary>
+    string EntityTypeName => typeof(TEntity).Name;
+}