OptionalhintsOptionalmetaFree-form per-item meta carried over from the zone's itemMeta map.
Strategies can read flags like pinned here; consumers set values via
store.setItemMeta / store.patchItemMeta.
OptionalplacementPer-membership placement intent projected from node.slot.placement.
size is the public "fixed-px pane" API: set it via store.patchPlacement
to pin a pane's main-axis extent. The strip / stack / split strategies
honor it; split's gutter drag clears it (reverting to ratio control).
Either Size dimension is optional.
Ceiling honored by the strip / stack / split strategies along their main axis (and by split's ratio/explicit clamping).