Seam over subprocess execution for outward-facing commands that have no dedicated facade of their own (git goes through GitCli; everything else that shells out to affect the world outside the BEAM - desktop notifications, escript installs - routes through this module). impl/0 resolves the :exec Globals key (same dispatch pattern as :http_client), defaulting to the System passthrough in the sibling Default module. Tests get the Mox mock with NO default stub, so a test that reaches an unscripted subprocess fails loudly instead of spawning one.