When adding a formula interactively, the target (e.g. "Margin") was
registered as a formula but never added as an item to the target
category. The grid layout never created cells for it, making the
formula invisible. Now AddFormula::apply calls add_item before
registering the formula.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>