Module guide

A module is a unit with an interface and an implementation; its interface includes invariants, ordering, errors, configuration, and performance obligations as well as types.
Prefer depth: useful behavior behind a small interface. Assess what callers must learn and where future changes will concentrate.
A seam is where the interface lives. Name the dependency that varies there and the concrete adapters that justify it; do not add a speculative port for one fixed implementation.
Test proposed behavior through the interface visible to callers. Identify the existing tests that would stay meaningful after an internal refactor.
Classify dependencies as in-process, locally substitutable, owned remote, or external before recommending an adapter or verification seam.
State what each candidate hides, what it exposes, and how an implementer can prove the selected behavior. A diagram or example may clarify a design, but it is not verification evidence.