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.

go.mod

@@ -71,6 +71,7 @@ require (
 	github.com/samber/slog-common v0.22.0 // indirect
 	github.com/spf13/afero v1.15.0 // indirect
 	github.com/spf13/pflag v1.0.10 // indirect
+	github.com/systemd/slog-journal v0.1.2 // indirect
 	github.com/valyala/bytebufferpool v1.0.0 // indirect
 	github.com/valyala/fasttemplate v1.2.2 // indirect
 	go.opentelemetry.io/auto/sdk v1.2.1 // indirect