Read DEFAULT-only ConfigParser configuration files

[harbinresearcher]

Fixes #361.

ConfigparserConfigFileParser.serialize() writes [DEFAULT], but parse() only reads ordinary sections. Saving and then loading a configuration therefore discards all settings when using this parser.

Read the default section when there are no ordinary sections. Keep the emitted format, named-section flattening/override order, interpolation, and value conversion unchanged. Clarify the syntax description and add five tests for DEFAULT-only values, serializer round trips, actual ArgumentParser.write_config_file() save/reload, named-section overrides, and empty inputs. Three new regressions fail before the fix; the other two are controls.

Validation:

• Windows CPython3.12.14 and Linux CPython3.9.25: available suite 159 -> 164 passed, with the same five expected failures.
• Linux CPython3.14.8 with matching official CPython v3.14.8 Lib/test supplied: the project's standard-library argparse compatibility adapter runs; full suite 2052 -> 2057 passed, one skip and five expected failures on both trees. Both runs have the same coverage dynamic-context warning.
• Black check passes on pristine and patched trees. CI's strict Pydoctor command passes on both with a task-local intersphinx cache. Zizmor reports no findings with the repository configuration (offline mode).
• Isolated sdist/wheel build succeeds; installed-wheel tests pass 164 cases with the same five expected failures on Windows.
• Independent automated read-only review found no actionable issues and independently ran the Windows tests.

The managed Windows and Linux3.9 interpreters lack test.test_argparse; the compatibility suite was run on Linux3.14 only. Python3.6-3.8, PyPy, macOS, and the full CI matrix were not run locally. No production incident or security claim.

Disclosure: this contribution was investigated, implemented, tested, and reviewed autonomously with AI under the contributor's authorization. Independent review was automated, not human approval; maintainer review is still required.