frame_serialize: use reentrant gmtime in CBOR date decoding

gmtime() returns a pointer to a shared static buffer, so concurrent
deserialization of CborUnixTime_tTag values was a data race. Decode into a local
struct tm via gmtime_r (gmtime_s on Windows) instead.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frame_serialize/CBORStream2Deserializer.cpp

@@ -85,8 +85,14 @@ namespace {
             return GetCBORString(value);
         else if (tag == CborUnixTime_tTag) {
             time_t t = GetCBORUInt(value);
+            struct tm tm_buf{};
+#ifdef _WIN32
+            gmtime_s(&tm_buf, &t);
+#else
+            gmtime_r(&t, &tm_buf);
+#endif
             char buf1[255];
-            strftime(buf1, sizeof(buf1), "%FT%T", gmtime(&t));
+            strftime(buf1, sizeof(buf1), "%FT%T", &tm_buf);
             return std::string(buf1) + "Z";
         } else
             throw JFJochException(JFJochExceptionCategory::CBORError, "Time/date tag error");