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.

README.md

@@ -133,6 +133,11 @@ the same manner to loggers in this library.
 
 ### LogFormatter
 
+Messages created by the library carry UTC timestamps in `LogMessage.ts`, which
+remains a `DateTime`. This changes the previous local-time behavior. Structured
+JSON output represents these timestamps with a `Z` suffix. Explicit timestamps
+in caller-constructed `LogMessage` values are preserved.
+
 Console, file, and std/logging appenders accept a *LogFormatter*. Noncapturing
 functions such as *formatSimpleTextLog* and *formatJsonStructuredLog* are
 converted automatically:
@@ -342,9 +347,8 @@ not refresh configuration; see [Avoiding Expensive Log Preparation](#avoiding-ex
 
 > [!NOTE]
 > Thread-safety work is still in progress. Custom state and captured
-> formatters must follow the ownership rules above. Concurrent local-time
-> timestamp construction still has a known failure.
-> Nim 2.2.10 also produces sanitizer reports in its default allocator
+> formatters must follow the ownership rules above.
+> Nim 2.2.10 produces sanitizer reports in its default allocator
 > during some cross-thread allocation patterns. See
 > [the regression tests](tests/README.md) for the remaining work.