FW-02: atomic_dump_yaml now calls _atomic_dump_yaml_check_nfs() which
detects NFS/CIFS/SSHFS mounts and warns that flock is unreliable.
Long-term fix (SQLite WAL) documented in FUTURE_WORKS.md.
FW-11: pyyaml added to requirements.txt and installed in .venv, so
both paho-mqtt and yaml are available in a single interpreter.
Eliminates the system-python3-vs-venv split for monitor --subscribe.