fix(log): Write each log line in one write
The server's logger and the session launcher's log wrote a line with
writeln!/eprintln! to stderr, which is unbuffered, so each formatted
fragment went out in a write of its own. The launcher and the server it
starts share one stderr (Jupyter's log, a journal), and their lines
interleaved mid-line: "[selkies-session] " ended a WARNING line and the
rest of the launcher's line began the next, so the Jupyter suite's check
for the "[selkies-session] desktop standin on wayland-1" line missed it.
Each line is formatted first and written whole, which a pipe keeps
together up to PIPE_BUF.