Fix review findings: stderr API, UDS perms, signal range, write loop,…

[nyo16]

Summary

Addresses findings from a parallel code review of the recent changes (commits #4–#6). One blocker, four warnings, and several cleanups.

Verified: mix format --check-formatted, mix compile --warnings-as-errors (clean C rebuild), mix credo --strict (no issues), mix test → 140 passed / 2 excluded.

Blocker

• Remove documented-but-unimplemented stderr: :redirect. Output was silently dropped, and a child emitting enough stderr could block on a full pipe. :stderr is now validated at spawn time (:consume /
  :disabled only); docs, typespec, and README updated.

Security / correctness

• nif_kill: reject signals outside 1..31 (mirrors the shepherd's CMD_KILL validation).
• UDS socket now lives in a per-spawn 0700 directory, blocking the cross-user SCM_RIGHTS FD-hijack vector. Also cleans up the dir on failure paths (previously leaked empty dirs in tmp).
  Peer-credential verification (SO_PEERCRED) deferred — platform-divergent.
• Write loop: a zero-byte write on a non-empty buffer is now mapped to EAGAIN + enif_select inside the NIF, dropping the GenServer-blocking Process.sleep(1) retry.
• Daemon drains via Task.Supervisor.async_nolink (added NetRunner.TaskSupervisor to the app tree) so a drain-task crash can't take the Daemon down.

Cleanups

• Log shepherd errors in handle_uds_message instead of dropping them.
• Operations: O(1) demonitor via an op_ref -> mref reverse index (was O(n) Enum.find).
• Command.new/3: guard clauses instead of unless/raise.

Tests

• process_test: poll instead of sleep(500) for async cleanup (CI flake risk).
• net_runner_test: unique pgrep -f marker — no longer racy under async: true.
• signal_test: assert_raise now matches the error message.
• daemon_test: added a drain-task isolation test.

Deferred (intentional)

• write_all_input error propagation — broken-pipe on stdin is normal for head-style commands; making it fatal would regress.
• cgroup symlink note (root/Linux, trusted config) and minor test-hygiene nits.