Use LF line endings for shell completion output

[felmonon]

Summary

• write shell completion source and completion output as bytes to bypass platform newline translation
• preserve LF line endings for zsh completion scripts on Windows/MSYS environments
• add a regression test that simulates a CRLF-translating text stdout and verifies zsh source output stays LF-only

Testing

• PYTHONPATH=src python3 -m pytest tests/test_shell_completion.py

Fixes #3277