Section and profile operations over a context:

  {:engine   IComposeEngine
   :runner   IProgramRunner                   ; optional: host programs
   :settings Settings
   :state    (atom {:active {id Active} :current id-or-nil})
   :now      (fn [] epoch-ms)
   :save!    (fn [persisted-state] Result)}   ; optional

A subject is a Profile: a configured preset (`lookup`) or a section resolved
from a target against the configured compose projects (`resolve-target`).
Every operation answers a Result (reap! and adopt! answer reports). The
active map changes only after the engine confirmed the action.

A section also holds the host programs paired with its services: they start
once its containers are up and stop when the last section holding them is
released. A program that fails to start is reported, never fatal.