feat(annotation): Add AnnotationOrigin value object

- Add AnnotationOrigin enum (BottomLeft/TopLeft)
- BottomLeft = PDF native coordinate system (default)
- TopLeft = UI-friendly coordinate system (requires Y-axis conversion)
- Shared with stamp operations for consistency

DocumentOperator.Domain/Models/ValueObjects/AnnotationOrigin.cs

@@ -0,0 +1,18 @@
+namespace DocumentOperator.Domain.Models.ValueObjects;
+
+/// <summary>
+/// Coordinate origin point for PDF annotations.
+/// PDF default is BottomLeft, but users may prefer TopLeft for easier UI integration.
+/// </summary>
+public enum AnnotationOrigin
+{
+    /// <summary>
+    /// Bottom-left corner (PDF native coordinate system, default)
+    /// </summary>
+    BottomLeft,
+    
+    /// <summary>
+    /// Top-left corner (common in UI frameworks)
+    /// </summary>
+    TopLeft
+}