Sentinel value returned from expect responders to delegate to the fallback/fake instead of returning a result directly.

Use DoubleDown.Double.passthrough/0 to create this value rather than constructing the struct directly.

When an expect responder returns this sentinel, the call is handled by the fallback (stateful fake, function fallback, or module fake) as if the expect had been registered with :passthrough. The expect is still consumed for verify! counting.

This enables conditional passthrough — the responder can inspect the state and decide whether to handle the call or delegate:

DoubleDown.Double.expect(:insert, fn [changeset], state ->
  if duplicate?(state, changeset) do
    {{:error, add_error(changeset, :email, "taken")}, state}
  else
    DoubleDown.Double.passthrough()
  end
end)