fix: ensure CommentReference character style is defined when adding a comment - #1610

What this fixes

Closes #1609.

Document.add_comment() writes <w:rStyle w:val="CommentReference"/> into the
reference run but never adds the matching style definition to styles.xml. The default
Document() template doesn't contain it either, so the reference mark ends up with a
dangling rStyle and falls back to default run formatting instead of the intended 8-pt
"annotation reference" appearance. Word and LibreOffice tolerate this silently, but the
mark is visually wrong and opens a subtle trap for any caller that inspects styles.

Fix

Styles._ensure_comment_reference_style() registers the built-in style on first use.
Document.add_comment() calls it before marking the comment range. The method is
idempotent — documents that already carry the style (e.g., re-opened Word files) are
left unchanged.

The style definition matches what Word 365 writes:

Changes

• src/docx/styles/styles.py: add _ensure_comment_reference_style() + Pt import
• src/docx/document.py: call self.styles._ensure_comment_reference_style() in add_comment()
• features/doc-add-comment.feature: new acceptance scenario
• features/steps/comments.py: given step for fresh document without the style + then assertion step

Testing

pytest tests/ -q -k "comment or style"
# 251 passed

Acceptance tests require behave (not available in this environment); the new scenario is
readable and can be verified by inspection.

Verification script:

from docx import Document

d = Document()
p = d.add_paragraph("Some text.")
print(d.styles._element.get_by_id("CommentReference"))  # None — not there yet

d.add_comment(runs=p.runs[0], text="A comment", author="Reviewer")
style = d.styles._element.get_by_id("CommentReference")
print(style.name_val)   # "annotation reference"
print(style.styleId)    # "CommentReference"
print(style.type)       # CHARACTER (2)