Fix SplitOnSections to properly handle body-level w:sectPr as section boundary

Previously, body-level w:sectPr elements were incorrectly included as
block-level content when computing section ranges. This caused the final
section's headers/footers/page settings to be merged into the previous
section when the last section was defined only by body-level section
properties.

The fix filters out w:sectPr elements from the body's child elements
before processing, since they are section properties rather than
block-level content. This ensures proper section boundary detection and
prevents sectPr elements from being counted in element ranges.

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

Author: knilecrack

OpenXmlPowerTools/DocumentBuilder.cs

@@ -242,10 +242,16 @@ public static IEnumerable<WmlDocument> SplitOnSections(WmlDocument doc)
         using OpenXmlMemoryStreamDocument streamDoc = new OpenXmlMemoryStreamDocument(doc);
         using WordprocessingDocument document = streamDoc.GetWordprocessingDocument();
         XDocument mainDocument = document.MainDocumentPart.GetXDocument();
-        var divs = mainDocument
+        // Filter out body-level w:sectPr elements as they are section properties, not block-level content
+        // A body-level sectPr marks a section boundary and should not be included in element counts
+        var bodyElements = mainDocument
             .Root
             .Element(W.body)
             .Elements()
+            .Where(e => e.Name != W.sectPr)
+            .ToList();
+
+        var divs = bodyElements
             .Select((p, i) => new Atbi
             {
                 BlockLevelContent = p,