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/writer_child.nim

@@ -12,9 +12,7 @@ proc formatOptionalLine(msg: LogMessage): string {.gcsafe.} =
 proc writeMessages(args: tuple[service: LogService, worker: int]) {.thread.} =
   let logger = args.service.threadLocalRef().getLogger("shutdown")
   for i in 0 ..< 2_500:
-    # Keep the timestamp race out of this writer lifecycle test.
-    logger.log(LogMessage(scope: "shutdown", level: lvlInfo,
-      msg: "message " & $(args.worker * 2_500 + i)))
+    logger.info("message " & $(args.worker * 2_500 + i))
 
 proc main() =
   doAssert paramCount() == 2