Exit codes and output are the contract

Wrappers can rely on three things: the exit code, the final STATUS: line, and ERROR: lines. rcp never prints STATUS: FAILED while exiting 0. The integration tests in tests/contract.rs pin this, and every arm that shows a check passing has a control arm that plants the defect and requires the check to fail. Sockets, FIFOs and device nodes are skipped with a WARN: line. A copy that fails on some entries still copies the rest, reports each failure, exits 1 and keeps its catalog, so resume can finish the job.