#4571 fix(pptx): read a line break as a newline instead of a vertical tab

Summary

python-pptx returns a line break inside a paragraph (Shift+Enter in PowerPoint, <a:br/> in the XML) as a vertical tab (\v). partition_pptx() passes paragraph.text through unchanged, so that control character ends up in the element text of titles, text boxes and slide notes:

source	element text on main	with this PR
title Quarterly ⏎ results	'Quarterly\x0bresults'	'Quarterly\nresults'
text box First line ⏎ second line	'First line\x0bsecond line'	'First line\nsecond line'
slide notes Note one ⏎ note two	'Note one\x0bnote two'	'Note one\nnote two'

Table cells were already unaffected, because their text goes through the HTML table normalization.

Change

A small _break_lines() helper replaces \v with \n. It is used for paragraph text in _iter_shape_elements() and _iter_title_shape_element(), and for the slide-notes text. A newline is how the DOCX partitioner already gives a w:br line break, since python-docx reads it as \n.

Testing

• New test_partition_pptx_reads_line_breaks_as_newlines builds a slide with a line break in the title, in a text box and in the notes. It fails on main.
• The other 73 tests in test_pptx.py pass. The .ppt tests need LibreOffice, which isn't installed here.
• ruff 0.15.10 check and format are clean, and scripts/version-sync.sh -c passes with 0.27.34-dev1.