The ADR-0027 session runtime: a module-based supervisor holding the library's named session registry and the flat dynamic supervisor sessions started through Statifier.start_session/2 land on.

The embedder places this, the library does not

Statifier-ex ships no mod: application callback (ADR-0027 decision 1). A library that started processes on load would tax every host that only ever compiles a document, and this module names the seam ADR-0003's "embedders can supply their own effect interpreter" already opened: an embedder adds Statifier.Supervisor (or {Statifier.Supervisor, []}) to their own supervision tree when they want cross-session #_scxml_<sessionid> delivery (via Statifier.start_session/2), and omits it entirely when they do not. A host that never places this supervisor starts no session-runtime processes and pays nothing.

Children, in order, under :rest_for_one

1. {Registry, keys: :unique, name: Statifier.Registry} - Session.init/1 registers under the sess_ id once it is generated (Session's own moduledoc, "<send> routing" section).
2. {DynamicSupervisor, name: Statifier.SessionSupervisor} - the flat supervisor Statifier.start_session/2 starts sessions on.

:rest_for_one means a registry crash takes the DynamicSupervisor (and every session under it) down with it rather than leaving live sessions behind a dead registry as unreachable orphans; ADR-0027 decision 1 records that those sessions restart fresh rather than continuing as amnesiacs (each session is restart: :temporary, so "restart" here means the embedder's own supervision tree deciding what happens next, not this supervisor bringing a session back - see decision 4). A DynamicSupervisor crash on its own does not take the registry with it, since registrations belong to the session processes, not to the supervisor that started them.

One instance, no :name option

ADR-0027 defers multiple named runtime instances as mechanism with no caller. Statifier.Registry and Statifier.SessionSupervisor are fixed, module-qualified names; starting a second Statifier.Supervisor in the same node is a name collision, not a second isolated session population. The test suite places one runtime for the whole run in test/test_helper.exs, for the same one-instance reason, rather than each test standing up its own registry/supervisor pair.