misc tuning, following hint by IntelliJ;
prefer fractional font metrics, for proper scaling of node size;
tuned metrics;
more explicit Layout.Info: size and content; allow multi-line vertex label, based on content; misc tuning;
explict layout graph structure, with dummies and coordinates; explicit metrics for dummy box; tuned signature; misc tuning;
separate module Metrics; maintain static metrics (with font) and visible_graph via layout;