The only effectful layer: run a plan through an injected elisp evaluator.

Every request is issued asynchronously and awaited under a deadline. A Slynk
request naming a callee the image cannot resolve is never answered, and a
synchronous wait on it blocks the whole editor; the deadline is what bounds
that to one failed verb.

The evaluator is injected, so tests drive this namespace with a recording
stub and never a live host.

Backend profiles and their registry — the DIP swap point.

A profile is plain data describing what one transport does with each verb.
Adding a third transport is `register!` of one more profile; nothing above
this namespace changes.

The :slynk profile's RPC names and argument lists were measured against a
running image, and the package qualifications are load bearing.

Malli value objects for the REPL-backend translation layer.

One vocabulary spans both transports: a caller names a VERB and a BACKEND,
and a backend PROFILE says which RPC that verb becomes. Everything here is
data — no protocol, no function, appears in a schema.

Pure translation: a verb-and-backend request becomes an executable plan.

Nothing here performs an effect or names a transport. Every backend
difference is read out of the profile, so a new backend needs no change in
this namespace.