What changed (2026-10-03, frankuser)
- Both scans are brace-balanced. At compile time the nested names are
renumbered with the outer field (PyFormatRenumberName, one helper for both);
at run time PyFormatExpandSpec substitutes each nested field's text into
the spec before it is applied. The outer field takes its automatic index
before the nested ones, as in CPython, so
"{:<{}} {:>{}.{}f}"reads name, width, value, width, precision in that order. - A
!rconversion with a spec formats the repr with the spec.
Test: test/test_nilpy_str_format_nests_a_field_in_its_spec.npy (automatic, numbered and named nested fields, a nested fill, a two-column table), byte-identical to CPython under -dPXX_HEAP_DEBUG.