OptionalisWhether the section is currently expanded. Optional: undefined means the user has
never explicitly expanded/collapsed this section, so the section's seeded default
(passed by the caller / from initSections()) should govern. This must stay decoupled
from panelHeight — persisting a height must NOT imply an expansion state, otherwise
measuring a collapsed panel on load would silently mark it expanded.
OptionalpanelUser-resized panel height in pixels (related-entity panels only)
State for an individual form section (panel)