sdd:Section
A sdd:Section is a flex region inside a sdd:Node: it holds either content — sdd:Datum and sdd:Port instances — or two-or-more child sections, never exactly one (design spec §1's all-or-nothing rule, applied recursively at every level of nesting). The node root is itself a specialized section, so the same flex model governs a node's outermost layout and its interior divisions alike.
Layout runs along the container's sdd:flow axis. A section's main-axis extent is either fixed — a percent or an absolute sdd:length — or a proportional sdd:weight share of what fixed sections leave behind. sdd:order materializes a section's 1-based declaration position: data the flex pass reads to decide reservation order, and exempt from the canonical reordering every other scene quad is subject to. A section may also sdd:rebinds its subject through a property path, changing what its own datums and ports bind relative to.