Function socket_dir 

pub fn socket_dir() -> String

Resolve the runtime socket directory. Priority: $XDG_RUNTIME_DIR/keepty → $TMPDIR/keepty (via std::env::temp_dir)

On macOS, $TMPDIR is already per-user (/var/folders/…/T/). On Linux, $XDG_RUNTIME_DIR is per-user (/run/user/$UID). No external dependencies — uses only std.