Builds the process-space polygons: for each stage level, the corridor
around the channel where the terrain stays below that stage rise. One
polygon layer per entry in transectObject$stageLevels (set by
addCrossSectionElevations).
addProcessSpace(transectObject)transectObject with $ProcessPolygons: a named list of sf
polygon layers, one per stage level (names like "1m", "2m").