test: add FuzzMarshal, golden messages, synctest cancellation and cross smoke

Assisted-by: GLM 5.3 Flash

datetime.go

@@ -258,6 +258,12 @@ func parseDateTime(tok string) (any, bool) {
 		if err != nil {
 			return nil, false
 		}
+		// A zero offset carries its own anonymous location from time.Parse,
+		// while the written form is "Z" either way; normalising to UTC keeps
+		// the tree identical across the round trip.
+		if _, off := t.Zone(); off == 0 {
+			t = t.In(time.UTC)
+		}
 		return OffsetDateTime{t}, true
 	case dateTimeLocal:
 		layout := localClockLayout