Atomic counter via SELECT FOR UPDATE on the parent SO row. Composes
child names as PREFIX-PARENT (bare for first) or PREFIX-PARENT-NN
(zero-padded 2-digit, then unpadded past 99). Subclasses implement
three hooks: _fp_parent_sale_order, _fp_name_prefix, _fp_parent_counter_field.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>