Summary

Adds first-class support for PDF annotation replies (PDF spec 1.5, Table 170) to the MarkupAnnotation base class.

New parameters on MarkupAnnotation

• in_reply_to: Reference to a parent annotation (the object returned by writer.add_annotation()). Sets the /IRT entry.
• reply_type: Either "R" (reply, default) or "Group" (grouped). Sets the /RT entry.
• annotation_name: Sets the /NM entry for annotation threading. Auto-generated as a UUID when in_reply_to is set and no name is provided.

Usage example

from pypdf import PdfWriter
from pypdf.annotations import Text

writer = PdfWriter()
writer.add_blank_page(width=612, height=792)

# Create parent annotation
parent = Text(text="Original comment", rect=(50, 700, 200, 750))
parent_ref = writer.add_annotation(0, parent)

# Create reply annotation
reply = Text(
    text="Reply to the comment",
    rect=(50, 700, 200, 750),
    in_reply_to=parent_ref,
)
writer.add_annotation(0, reply)

with open("annotated.pdf", "wb") as f:
    writer.write(f)

Design notes

• Follows the same pattern as Popup(parent=...) for referencing other annotations via indirect_reference
• Warns (without crashing) when in_reply_to receives an unregistered annotation object, matching the existing Popup behavior
• All existing MarkupAnnotation subclasses require rect, so the workaround discovered in the issue thread (reply annotations need /Rect to be visible) is handled naturally

Tests

5 new tests covering:

• Basic reply with /IRT, /RT, /NM verification + round-trip read
• reply_type="Group" variant
• Explicit annotation_name without in_reply_to
• Custom annotation_name with in_reply_to
• Graceful warning for unregistered parent annotations

Closes #2065