Drop the false reason from present_main's single read of the trace flags.

The review found that the comment in vk_backend.c present_main and the message of 3236dc3c gave a cause that cannot
happen: the menu cannot turn trace=perf on between the two reads. rt_trace is set once at start (main.c parse_trace),
TRACE is a RESTART row in cfg.c metas, no menu code changes it, and the only other writer, ipc.c probe_sdk_client,
runs in a probe mode and changes only the IPC bit. The single read stays, since the stamps and both lines then follow
the same flags; the comment now says only that.

Comment only. vk_backend.c compiles with the dev flags and with -Wall -Wextra without new warnings.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

runtime/vk_backend.c

@@ -110,7 +110,7 @@ static void present_main(void *arg) {
         busy = 1;
         plat_cond_broadcast(&pcv);
         plat_mutex_unlock(&pl);
-        unsigned tr = rt_trace;   // once, as the menu's performance log can switch the trace meanwhile
+        unsigned tr = rt_trace;   // read once: the stamps and both lines below follow the same flags
         int timed = (tr & (RT_TRACE_PRESENT | RT_TRACE_PERF)) != 0;
         uint64_t b0 = timed ? kern_now_ns() : 0;
         if (flight_on || vk_dump_white) vk_thumbs_frame(j.frame);   // NX2NAVI_DUMP_WHITE reads them too