Preserve input line endings in output file

[wegank]

Fixes #299.

msolve writes its output file in binary mode, so the output always uses \n line endings, whatever the platform or the input. Users working with \r\n input files then have to run unix2dos on the output themselves.

As suggested in #299, this PR detects the line ending of the input file before core_msolve in src/msolve/main.c. If the input uses \r\n, the output file (-o) is rewritten afterwards with \r\n line endings. This adds no overhead for inputs with \n line endings.

• The line ending is detected from the first line of the input file.
• Each \n not already preceded by \r becomes \r\n. The file is copied to a tmpfile() and rewritten in place, so its permissions and any symlink are kept. If the conversion fails, msolve prints a warning and the output keeps \n line endings.
• Output to stdout (no -o) is not converted.

Tests

The new test test/diff/diff_line-endings.sh runs msolve on the pairs of inputs in input_files/line_endings/, which differ only by their line endings, with default options, -g 2 and -P 1. It checks that:

• the output for the Unix input contains no \r, and
• the output for the DOS input is byte-identical to the output for the Unix input with \r\n line endings.

The existing line_endings_support test still checks that the parser reads both inputs identically. make check passes locally on macOS (69/69). This has not been tested on Windows.

AI disclosure

This PR was written with the assistance of generative AI (Claude Code), and reviewed by me.

🤖 Generated with Claude Code