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_test.go

@@ -0,0 +1,28 @@
+package logger
+
+import "testing"
+
+func TestSanitizeJournalKey(t *testing.T) {
+	tests := []struct {
+		in, want string
+	}{
+		{"", ""},
+		{"MESSAGE", "MESSAGE"},
+		{"trace_id", "TRACE_ID"},
+		{"span.id", "SPAN_ID"},
+		{"http-method", "HTTP_METHOD"},
+		{"a", "A"},
+		{"123abc", "_123ABC"},
+		{"weird!@#key", "WEIRD___KEY"},
+		{"_leading", "_LEADING"},
+		{"MiXeD_Case", "MIXED_CASE"},
+	}
+	for _, tt := range tests {
+		t.Run(tt.in, func(t *testing.T) {
+			got := sanitizeJournalKey(tt.in)
+			if got != tt.want {
+				t.Errorf("sanitizeJournalKey(%q) = %q, want %q", tt.in, got, tt.want)
+			}
+		})
+	}
+}