Add comments, highlights and drawing annotations

[McanKul]

Goal

Let users review a PDF with standard annotations that remain inspectable in compatible readers.

Scope

• Create text-note, highlight, underline, strikeout and ink annotations.
• Provide author, color and optional comment text.
• List annotations for the current document and allow deleting annotations created in the current session.
• Preserve existing annotations during save.
• Offer an explicit Flatten annotations option for users who need fixed visual output.

Out of scope

• Real-time review collaboration.
• Importing replies or full annotation-thread editing.
• Cryptographic signatures.

Acceptance criteria

• Saved annotations appear in at least two common PDF viewers.
• Highlight geometry is stored as QuadPoints, not only a bounding rectangle.
• Existing annotations survive a no-op save.
• Flattening is opt-in and produces the same visible result.
• Rotated/CropBox pages place annotations correctly.
• Tests cover supported annotation dictionaries, appearance streams and malformed annotations.

Technical note

lopdf can inspect page annotations; qpdf can flatten annotations, but interactive annotations require correct dictionaries and appearance streams.

[McanKul]

Completed in #63 and merged into development. Thanks for the contribution.