An unannotated override of a Pascal virtual widens the compiled base

Found building songformatter (2026-10-27, peer session):

class CP(configparser.ConfigParser):
    def optionxform(self, optionstr):
        return optionstr
cfg = CP(); cfg.add_section("Render"); print(cfg.has_option("Render","Font"))

segfaulted in has_option (repz cmpsb). A plain ConfigParser printed False.

Cause. The override block in PyRegisterClassMembers joins a variant override with an AnsiString base through PyOverrideRetJoinsToVariant, and it does that by setting the BASE's result to Variant. That is sound for a Python base, because the pre-pass runs before the base body is compiled. configparser.pas is compiled Pascal whose has_option calls optionxform through the slot and reads an AnsiString. test_nilpy_configparser.npy annotates s: str -> str, so it never took the join.

Fix. The join (and the widening) applies only when the base class is Python (a py module, the main program, or a pylib class, which keeps its long-standing behaviour). A Pascal-unit base keeps its declared signature, and the override adopts it.

Test: test/test_nilpy_an_unannotated_override_of_a_pascal_virtual.npy (CPython oracle). Pin v452 fails it.