Create log timestamps in UTC to avoid local-time races

Use getTime().utc in both message constructors, document the timezone change, and cover concurrent construction and structured formatting. Restore ordinary logging calls in the parallel writer fixture.

Validation: 32 unit tests and 29 regression tests pass. Timestamp sanitizer checks pass under UTC and non-UTC timezones. The full suite retains the known allocator reports in factory-state and allocator.

tests/thread_safety.nim

@@ -158,8 +158,13 @@ proc factoryWorker(source: ptr LogService) {.thread.} =
 proc timestampWorker() {.thread.} =
   awaitStart()
   for i in 0 ..< iterations:
-    let msg = initLogMessage("worker", lvlInfo, "message")
-    doAssert msg.msg == "message"
+    for msg in [initLogMessage("worker", lvlInfo, "message"),
+        initLogMessage("worker", lvlInfo, %*{"msg": "message"})]:
+      doAssert msg.msg == "message"
+      doAssert msg.ts.timezone.name == "Etc/UTC"
+      doAssert msg.ts.utcOffset == 0
+      doAssert not msg.ts.isDst
+      doAssert parseJson(formatJsonStructuredLog(msg))["ts"].getStr.len > 0
 
 proc collectorWorker(collector: LoggedMessages) {.thread.} =
   # Construct every managed field in this worker, including the timezone.