Insert
@Serializable
Insert value with identity id.
The Fugue tree placement is specified by parent and side:
parent is the tree node this element is a child of (not the sequence left-neighbour — the tree parent is computed from the sequence context at insert time).
side = FugueSide.Left: left child of parent (inserted after a descendant of parent).
side = FugueSide.Right: right child of parent (inserted into parent's open right slot). rightOrigin records the next element in the traversal at insert time, used to sort right siblings.