Fix backslash round trips in set_key

[mjq2020]

Summary

• escape backslashes before serializing single-quoted values in set_key
• keep quote escaping behavior while making Windows paths, regexes, and repeated backslashes round-trip
• add regression coverage for paths, regexes, repeated backslashes, and adjacent quotes

Root cause

set_key escaped only single quotes, while the single-quoted parser decodes both \\ and \'. Consecutive backslashes were therefore collapsed when the file was read back.

Tests

• .venv-task/bin/ruff format --check src tests
• .venv-task/bin/ruff check .
• .venv-task/bin/pytest tests/test_main.py -q (119 passed)
• PATH="$PWD/.venv-task/bin:$PATH" .venv-task/bin/pytest -q --ignore=tests/test_cli.py (187 passed)
• PATH="$PWD/.venv-task/bin:$PATH" .venv-task/bin/pytest tests/test_cli.py -q -k "not test_run_with_command_flags" (39 passed, 1 deselected)

The excluded CLI test invokes GNU-only printenv --version, which is unsupported by macOS BSD printenv; it is unrelated to this change.

Fixes #661