Port-aware layout of untyped string diagrams (SDiagram).

Coordinates use chart-svg data space (y increases upward). Morphisms compose left-to-right (SThenD); tensor stacks top-to-bottom (SBeside). That matches the left/right port model and the wall-chart schematics — not the wording slip in the requirements bullet that had composition stacking vertically.

Every primitive emits its boundary ports on a uniform vertical grid with portPitch spacing, so thenL aligns whole port lists pairwise and besideL concatenates port lists slot by slot. A port-count mismatch in thenL is a wiring error: the connection is truncated to the common prefix and each unmatched port is marked with a dangling stub and dot, so the error is visible rather than silent.