vendor: import KillerPDF 1.7.5 source (GPL-3.0) as the base for MMD PDF

Models/UndoTypes.cs

@@ -0,0 +1,28 @@
+using System.Collections.Generic;
+
+namespace KillerPDF
+{
+    // The undo stack's entry type. Each entry is either an annotation removal or a full document
+    // snapshot; AnnotationGroup removes a specific set in one step (a text edit = cover + text).
+    //
+    // TOP-LEVEL, not nested in MainWindow. The code that pushes undo entries - Annotations.cs and
+    // TextEditing.cs - lives in KillerPDF.Controls, where a type nested in MainWindow only spells
+    // as MainWindow.UndoEntry; that would mean qualifying roughly 30 call sites for no gain. As
+    // top-level types in KillerPDF they resolve unqualified from the child namespace too.
+    //
+    // This also retires the CS0052 chain that made them internal in the first place: DocumentSession
+    // had to be internal for the render cache, its UndoStack field is Stack<UndoEntry>, and a field
+    // cannot be more accessible than its type.
+
+    internal enum UndoKind { Annotation, Document, StampBatch, ClearAnnotations, AnnotationGroup, PageSnapshot }
+
+    internal readonly record struct UndoEntry(
+        UndoKind Kind,
+        int PageIdx = -1,
+        byte[]? DocBytes = null,
+        bool WasDirty = false,
+        int[]? Pages = null,
+        PageAnnotation? Annot = null,
+        Dictionary<int, List<PageAnnotation>>? AnnotSnapshot = null,
+        List<PageAnnotation>? AnnotGroup = null);
+}