- dividers.rs: ratio/pending/echo-suppression state machine with no
  GTK types; App feeds observations and executes Place/Send commands.
  Settle/echo logic covered by headless unit tests.
- actions.rs: window actions defined once (name, menu label+section,
  accelerator); menu model, gio install and accels all build from the
  table, with consistency tests incl. a headless gio::Menu check.