A generator's non-constant default lands one parameter slot early

def g(a, b=None, c=None, d=acc): the generator read acc for c (CPython: None). Silent wrong value. It regressed in 9346fa34f9 (2026-10-09, found by bisect 2026-10-19) and broke test_nilpy_a_generator_takes_its_defaults_and_a_generator_method_is_a_value. Nobody saw it because the Nil Python recipe stopped at row 593 from 10-02 on (bug-tests-test-nilpy-stopped-at-a-vacuous-leak-row).

Cause: PyDeclareDefDefaults (the def-shell pre-declaration of non-constant defaults) indexed ProcParamDefaultSym by source position. A generator's ABI puts the instance in slot 0 and shifts every parameter up by one (PyApplyGeneratorABI).

Fix: a slotBase argument, 1 for a generator and 0 otherwise.

Test: test/test_nilpy_a_generator_default_lands_in_its_own_slot.npy. Both affected older rows pass, with and without HEAP_DEBUG.