v2.1.0 — markup annotations + radio/flatten fixes

Minor, additive release.

Added

• Markup annotations on existing documents (PdfDocumentEditor.Pages) and when authoring new ones (PdfPageBuilder): AddSquare, AddCircle, AddLine, AddInk, AddFreeText. Each generates a real /AP appearance stream, so it renders in this library''s renderer and in external viewers.
• /AP generation for the existing Highlight and Note add paths.
• Richer annotation reader: PdfAnnotationInfo now exposes AnnotationId, StrokeColor, InteriorColor, BorderWidth, LineEndpoints, InkPaths, Quadding, DefaultAppearance.
• PdfPageCollection.RemoveAnnotation(page, annotationId) — identity-based delete.

Fixed

• PdfRadioGroupBuilder now emits valid radio fields (parent /Btn + per-option widgets with vector-circle /AP); previously they read back as Unknown.
• FormFlattener now bakes state-keyed checkbox/radio /AP and removes the widget, fixing orphaned widgets that vanished after an Acrobat resave.

Published packages: Lxman.PdfLibrary 2.1.0 and Lxman.PdfLibrary.Rendering.Wpf 2.1.0.