[Bug]: HWPX → TXT/MD drops line breaks inside a paragraph (Unhwp ignores <hp:lineBreak/>) #1416

What happened?

When a paragraph has line breaks (<hp:lineBreak/>), HWPX → TXT and HWPX → MD put the whole paragraph on one line. The text, the indentation and the empty lines are there in the HWPX but come out joined:

import java.io.BufferedReader;class FileTask implements Runnable {    private String fileName;    public void run() {        int count = 0; …

The HWPX itself is right. This one came from RTF → HWPX (Pandoc reads the RTF's line breaks as LineBreak, and the HWPX writer turns them into <hp:lineBreak/>), and its section0.xml has one paragraph with 13 of them:

<hp:t>import java.io.BufferedReader;<hp:lineBreak/><hp:lineBreak/>class FileTask implements Runnable {<hp:lineBreak/>    private String fileName;<hp:lineBreak/> …

The line breaks get lost in Unhwp: UnhwpConverter takes the text from document.ToText() (TXT) and Unhwp's Markdown (MD), and both drop <hp:lineBreak/>. The same file through Unhwp's Python package gives the same one line from extract_text and to_markdown, both in 0.13.0 (the version Filee uses) and in the latest 0.23.1.

Found while working on #1204 / #1388, where it was the reason RTF → TXT couldn't go through the HWPX route.

Any HWP/HWPX file with line breaks inside a paragraph is affected, not only files made from RTF.

What did you expect?

Each hp:lineBreak/ becomes a line break in the text (in Markdown, a hard break), so lines, indentation and empty lines stay as they are.

Steps to reproduce

1. filee convert sample.rtf --to hwpx (the attached RTF; any RTF with several lines in one paragraph, e.g. code from TextEdit, works).
2. Check Contents/section0.xml inside the HWPX: the <hp:lineBreak/> elements are there.
3. filee convert sample.hwpx --to txt (or --to md): everything is on one line.

Without Filee, the same with Unhwp's Python package (0.23.1):

python3 -c "import unhwp; print(repr(unhwp.extract_text('sample.hwpx')))"

Files involved

sample.rtf (the RTF) and sample.hwpx (what Filee writes from it), both attached.

lineBreak-sample.zip

Filee version

main (47e24cd), also 2.3.0

Operating system

macOS 27.0.1

Engines page status

Built-in Unhwp; Pandoc found on this Mac (Homebrew), needed only for the RTF → HWPX step.

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