frame_children_mutation_test: validates remove(), property mutation via
stored refs, fill_color persistence, iteration mutation, pop(), and
while-loop clearing — with visual screenshot verification.
parent_none_removal_test: validates that setting .parent = None removes
children from Frame.children and scene.children, Entity.grid = None
removal, and Grid overlay children removal.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
