message: cache color settings before async start

The async log drain could be the first caller to inspect NO_COLOR and TERM while startup was still able to clear MAGIC through file_init(). Environment reads and updates are not safely synchronized.

Resolve and cache the stderr color decision in message_async_start() before creating the drain thread. Subsequent drain output uses the cached result, avoiding concurrent environment access without changing startup ordering.

Author: stevegrubb

src/library/message.c

@@ -464,6 +464,11 @@ int message_async_start(void)
 				 memory_order_relaxed) != LOG_ASYNC_OFF)
 		return 0;
 
+	/* file_init() clears MAGIC later in startup. Cache the stderr color
+	 * environment before the drain thread can read it concurrently. */
+	if (message_mode == MSG_STDERR)
+		(void)stderr_color_enabled();
+
 	if (sem_init(&log_sem, 0, 0))
 		return errno;