feat(01-02): add SharePointPaginationHelper, ExecuteQueryRetryHelper, LogPanelSink

- SharePointPaginationHelper: async iterator with ListItemCollectionPosition loop (bypasses 5k limit); RowLimit=2000; [EnumeratorCancellation] for correct WithCancellation support
- ExecuteQueryRetryHelper: exponential backoff on 429/503/throttle; surfaces retry events via IProgress<OperationProgress>; max 5 retries
- LogPanelSink: custom Serilog ILogEventSink writing color-coded entries to RichTextBox via Dispatcher.InvokeAsync for thread safety

SharepointToolbox/Core/Helpers/SharePointPaginationHelper.cs

@@ -0,0 +1,56 @@
+using System.Runtime.CompilerServices;
+using Microsoft.SharePoint.Client;
+using SharepointToolbox.Core.Models;
+
+namespace SharepointToolbox.Core.Helpers;
+
+public static class SharePointPaginationHelper
+{
+    /// <summary>
+    /// Enumerates all items in a SharePoint list, bypassing the 5,000-item threshold.
+    /// Uses CamlQuery with RowLimit=2000 and ListItemCollectionPosition for pagination.
+    /// Never call ExecuteQuery directly on a list — always use this helper.
+    /// </summary>
+    public static async IAsyncEnumerable<ListItem> GetAllItemsAsync(
+        ClientContext ctx,
+        List list,
+        CamlQuery? baseQuery = null,
+        [EnumeratorCancellation] CancellationToken ct = default)
+    {
+        var query = baseQuery ?? CamlQuery.CreateAllItemsQuery();
+        query.ViewXml = BuildPagedViewXml(query.ViewXml, rowLimit: 2000);
+        query.ListItemCollectionPosition = null;
+
+        do
+        {
+            ct.ThrowIfCancellationRequested();
+            var items = list.GetItems(query);
+            ctx.Load(items);
+            await ctx.ExecuteQueryAsync();
+
+            foreach (var item in items)
+                yield return item;
+
+            query.ListItemCollectionPosition = items.ListItemCollectionPosition;
+        }
+        while (query.ListItemCollectionPosition != null);
+    }
+
+    private static string BuildPagedViewXml(string? existingXml, int rowLimit)
+    {
+        // Inject or replace RowLimit in existing CAML, or create minimal view
+        if (string.IsNullOrWhiteSpace(existingXml))
+            return $"<View><RowLimit>{rowLimit}</RowLimit></View>";
+
+        // Simple replacement approach — adequate for Phase 1
+        if (existingXml.Contains("<RowLimit>", StringComparison.OrdinalIgnoreCase))
+        {
+            return System.Text.RegularExpressions.Regex.Replace(
+                existingXml, @"<RowLimit[^>]*>\d+</RowLimit>",
+                $"<RowLimit>{rowLimit}</RowLimit>",
+                System.Text.RegularExpressions.RegexOptions.IgnoreCase);
+        }
+        return existingXml.Replace("</View>", $"<RowLimit>{rowLimit}</RowLimit></View>",
+            StringComparison.OrdinalIgnoreCase);
+    }
+}