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/regressions.nim

@@ -1,7 +1,27 @@
-import std/[os, osproc, strutils, tempfiles, unittest]
+import std/[json, os, osproc, strutils, tempfiles, times, unittest]
 import ../src/namespaced_logging
 import ./ownership
 
+suite "Log message timestamps":
+  test "both constructors capture the current instant in UTC":
+    let before = getTime()
+    let textMessage = initLogMessage("timestamp", lvlInfo, "text")
+    let jsonMessage = initLogMessage("timestamp", lvlInfo, %*{"msg": "json"})
+    let after = getTime()
+    for message in [textMessage, jsonMessage]:
+      check message.ts.timezone.name == "Etc/UTC"
+      check message.ts.utcOffset == 0
+      check not message.ts.isDst
+      check message.ts.toTime >= before
+      check message.ts.toTime <= after
+
+  test "structured output represents generated timestamps in UTC":
+    for message in [initLogMessage("timestamp", lvlInfo, "text"),
+        initLogMessage("timestamp", lvlInfo, %*{"msg": "json"})]:
+      let encoded = parseJson(formatJsonStructuredLog(message))["ts"].getStr
+      check encoded == message.ts.format("yyyy-MM-dd'T'HH:mm:sszzz")
+      check encoded.endsWith("Z")
+
 type
   CloneControl = object
     failNext: bool