A lambda capturing a str could not call one of the program's defs

Found 2026-10-20 in Vloot's hover rows: lambda e: paint(hover_bg) where hover_bg is a str parameter answered pyeval: unknown call: paint() on every <Enter>.

Cause: the lambda lifter refused to lift a lambda that captures a managed string (a bound slot holds an Int64, and a borrowed string handle would outlive its owner; bug-nilpy-lifted-lambda-cannot-capture-a-managed-string). Such a lambda fell back to the pyeval source closure, which handles expressions but knows no user def.

Fix: the string is captured in a VARIANT slot instead. pyboundfn_bind_var copies it into a slot the bound object owns, and the lifted proc declares that parameter a variant. The lambda is compiled, so it can call defs, and the string lives as long as the closure. The fallback remains for when pyboundfn_bind_var is not loaded or the capture slots are full.

Test: test_nilpy_lambda_capturing_a_str_calls_a_def (the pin prints pyeval: unknown call: tint()).