test: add backwards-compatibility guardrails

API-surface snapshot (public names + parameter names/kinds across 14
modules, stable across Python versions), subclass-compat suite covering
old-style explicit-parent-call and cooperative-super() third-party
widgets and bars, and exact render goldens. The super()-style width
kwargs test is a strict xfail documenting the known chain break that
the cooperative-super() migration will fix.

Author: wolph