feat(logger): deliver stderr to systemd journal with per-record priority

When stderr is connected to journald, switch from the plain text handler
to the native journal protocol via github.com/systemd/slog-journal. Each
record now carries PRIORITY, MESSAGE, and slog attributes as structured
fields, so journalctl -p and LogLevelMax= in unit files filter by
severity instead of dropping everything at once.

Detection follows systemd.exec(5): parse JOURNAL_STREAM as <dev>:<ino>
and fstat(2) stderr to confirm the match, guarding against processes
that inherit the env var but have stderr redirected elsewhere. The
fstat path is Linux-only; other platforms fall through to the existing
text handler.

Also honour DEBUG_INVOCATION by raising stderr to Debug level, matching
the behaviour of systemd.service(5) RestartMode=debug.

logger/journald_other.go

@@ -0,0 +1,8 @@
+//go:build !linux
+
+package logger
+
+// stderrIsJournal always returns false on non-Linux platforms. The
+// systemd journal is Linux-only; developer workstations and other
+// targets fall back to the stderr text handler.
+func stderrIsJournal() bool { return false }