feat(domain): Add annotation value objects for Feature 6

- Add AnnotationType enum (TextMarkup, FreeText, StickyNote, Circle, Square)
- Add TextMarkupStyle enum (Highlight, Underline, Strikeout)
- Support 5 annotation types as per CONTROLLER_ENDPOINTS.md requirements

DocumentOperator.Domain/Models/ValueObjects/AnnotationType.cs

@@ -0,0 +1,32 @@
+namespace DocumentOperator.Domain.Models.ValueObjects;
+
+/// <summary>
+/// Supported PDF annotation types
+/// </summary>
+public enum AnnotationType
+{
+    /// <summary>
+    /// Text markup annotation (highlight, underline, strikeout)
+    /// </summary>
+    TextMarkup,
+    
+    /// <summary>
+    /// Free text annotation (text box with visible text)
+    /// </summary>
+    FreeText,
+    
+    /// <summary>
+    /// Sticky note annotation (popup comment icon)
+    /// </summary>
+    StickyNote,
+    
+    /// <summary>
+    /// Circle shape annotation
+    /// </summary>
+    Circle,
+    
+    /// <summary>
+    /// Square shape annotation
+    /// </summary>
+    Square
+}

DocumentOperator.Domain/Models/ValueObjects/TextMarkupStyle.cs

@@ -0,0 +1,23 @@
+namespace DocumentOperator.Domain.Models.ValueObjects;
+
+/// <summary>
+/// Text markup annotation style (highlight, underline, strikeout)
+/// Maps to DevExpress PdfTextMarkupAnnotationType
+/// </summary>
+public enum TextMarkupStyle
+{
+    /// <summary>
+    /// Highlight text with background color
+    /// </summary>
+    Highlight,
+    
+    /// <summary>
+    /// Underline text
+    /// </summary>
+    Underline,
+    
+    /// <summary>
+    /// Strikeout text (strikethrough)
+    /// </summary>
+    Strikeout
+}