Lifted from fewo-webapp (src/App.cpp). 15-line helper that speaks the
systemd notification protocol directly — no libsystemd link — for
Type=notify services.
Silent no-op when NOTIFY_SOCKET is unset so the same binary runs
unchanged under systemd or as a plain background process.
Supports Linux abstract-namespace sockets.
Unblocks fewo-webapp #451 and its twin extractions for derived projects.
