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/README.md

@@ -54,9 +54,11 @@ checks that each worker maintains its own counter. `collector` concurrently
 records messages containing JSON and exceptions and mutates returned snapshots.
 These ownership scenarios use explicit timestamps where messages are needed.
 
-The separate `timestamp` case exercises concurrent `initLogMessage()` calls. It
-currently exposes a sanitizer report in libc local-time conversion reached via
-Nim's `std/times.now()`; this remains a separate thread-safety issue.
+The separate `timestamp` case exercises both `initLogMessage()` overloads and
+structured JSON formatting concurrently. Generated timestamps use UTC, avoiding
+the libc local-time conversion race previously reached via `std/times.now()`.
+The timestamp regressions verify the captured instant, UTC timezone, and `Z`
+suffix in structured output.
 
 `allocator` calls the standalone reproduction in `runtime_allocator.nim`, which
 imports no logging code. It exercises sequence growth and clearing under a lock
@@ -117,9 +119,8 @@ The suite provides targeted regressions; it does not exhaust thread scheduling
 or ORC cycle-collector interleavings.
 
 The ownership scenarios, lifetime suite, and appender-state cleanup regression
-exercise the new ownership contracts. Local-time timestamp construction and
-allocator reports remain for
-subsequent work.
+exercise the new ownership contracts. Allocator reports remain for subsequent
+work.
 
 The standalone `ownership.nim` regression suite passed AddressSanitizer and
 LeakSanitizer with `--mm:orc -d:useMalloc`. An additional worker-thread build found