baba5e0 replaced _advance_child with _advance(index, parent_buffer) driven
from _hop; the two feed-order tests still patched the removed method.
Patch _advance and record only index 2 (L1 hop feeding L2); assertions,
expected order and the stale-feed negative control are unchanged.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>