Listbox label, empty-state placeholder, create-new escape hatch with
noopener target, populated list, default aria-selected on first item,
life-date range visibility, position fallback when clientRect is null,
positioning from clientRect.
8 tests covering ~25 branches.
Refs #496.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
