A lambda's object capture dangled once the enclosing call returned

Found 2026-10-20 in Vloot's hover rows, once the lambdas were compiled (see bug-nilpy-a-lambda-capturing-a-str-cannot-call-a-def): clickable([row, title], ...) binds lambda e: paint(hover_bg) and returns. On <Enter>, paint walked widgets (a list) that had already been freed (rc 139; len(widgets) read 1634887022).

Cause: the lambda lifter bound a class-typed capture with the plain pyboundfn_bind, which takes no reference. A nested def taken as a value already used the retaining pyboundfn_bind_obj (PyNestedDefClosureValue).

Fix: the lifter binds tyClass captures through pyboundfn_bind_obj, whose finaliser releases the reference with the closure.

Test: test_nilpy_lambda_capturing_a_str_calls_a_def (hover_rows) and examples/tk/sidebar_cards_and_hover_rows.