Detecting whether a Python was built by `python-build-standalone`

Due to #380, I'm looking for a way to detect whether a given Python interpreter was built by `python-build-standalone`. Is there any reliable metadata (maybe something on `sysconfig.get_config_vars()`) to support this?

Review issue #380 for the motivation, then investigate whether Python's sysconfig.get_config_vars() or another interpreter-level metadata source can reliably identify python-build-standalone builds. Compare the available signals across supported builds and document a concrete detection approach or explain why no reliable mechanism exists.

Written by the indexing model from the issue text.