0.18.0

Added

• A specification's scenarios can be rendered as a page somebody presses play on.
  ess verify conform web emits a player: the scenarios on the left, a swimlane in the middle with
  one lane per actor and one row per act, and the state, the views and an optional device surface on
  the right. It is emitted for any specification and knows nothing about any of them — the page and
  the engine are static assets, and the only generated file is model.json, the projection the page
  reads.

  The reason to have it is that a specification nobody has run is a specification nobody has checked,
  and until now the only way to run one was to hand-write an implementation of it. Two exist —
  reference::Billing and reference::Oracle — and they exist so a generated suite has a
  known-good target, not so that every adopter writes one before seeing anything move.

  It replays rather than executes, and says so on the page. A scenario declares which outcome
  each command took; the player applies the effect the model attaches to that outcome. Every
  transition it shows is the model's — an outcome names its transition and the states it runs from —
  so a walk that stays legal says the specification is coherent, never that an implementation works.

  Three things the page keeps apart, because the model does: state is the truth now, a view
  is a projection with a filter, parameters and a consistency, and the UI is whatever a hand-
  written skin.js beside the emitted files renders. A view that selects nothing shows no rows; one
  whose parameter nothing has bound says which parameter it wants; an eventual one is allowed to be
  behind the state panel next to it.

  A binding's consequence is drawn in a lane of its own, dashed, because the model declares it and no
  scenario asserts it. That lane is where a command runs on nobody's grant, which is visible rather
  than argued about.

  assets/vue.esm-browser.prod.js is vendored unmodified with its licence beside it, the way
  assets/mermaid.min.js already is. No package.json enters the repository.