A real, in-memory Managoat.Sandbox adapter.

It lives under lib/, not test/support, on purpose: it is compiled in every environment so that a host application's own suite can run Managoat.Sandbox.ConformanceCase against it, drive a provisioning path without a network, or register it in the adapter map — the same way Ecto.Adapters.SQL.Sandbox ships inside Ecto rather than beside it.

Three jobs:

• prove the conformance suite is not Sprites-shaped — a second adapter has to pass it;
• exercise the owner-message contract for real — commands are actual processes emitting the {:stdout | :stderr | :exit, %{ref: ref}, _} frames, where most tests hand-craft them and can silently drift;
• be the reference implementation the next adapter's author reads.

Commands speak a tiny scripted vocabulary instead of a shell — each arg is an instruction:

• "out:<data>" / "err:<data>" — emit a stdout/stderr frame
• "exit:<code>" — emit the exit frame and stop
• "stay" — keep running: echo stdin writes back as echo:<data> stdout frames, exit 0 on stdin EOF
• "drop" — the transport closes without an exit frame; per the contract the adapter surfaces {:error, %{ref: ref}, :closed_before_exit}, and exec/4 the matching {:error, {:unavailable, :closed_before_exit}}

A script with no terminal instruction exits 0. State lives in a named Agent; call reset/0 in a setup block. Sessions buffer every emitted frame, and attach/3 replays the buffer from the start before tailing — the load-bearing semantic the conformance suite pins.