Render thread: user-interactive QoS on macOS

A default-QoS pthread is scheduled onto efficiency cores when the host is
busy, which put the render thread, now on every frame's critical path,
behind the emulation thread it is supposed to relieve.

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

runtime/src/render_thread.c

@@ -36,6 +36,9 @@ static void cpu_relax(void) { YieldProcessor(); }
 #else
 #  include <pthread.h>
 #  include <sched.h>
+#  if defined(__APPLE__)
+#    include <pthread/qos.h>
+#  endif
 #  include <time.h>
 typedef pthread_mutex_t rt_mutex;
 typedef pthread_cond_t  rt_cond;
@@ -392,7 +395,16 @@ int rt_start(const RtConfig *cfg) {
     R.thread = CreateThread(NULL, 0, render_main, NULL, 0, NULL);
     int ok = R.thread != NULL;
 #else
-    int ok = pthread_create(&R.thread, NULL, render_main, NULL) == 0;
+    pthread_attr_t attr;
+    pthread_attr_init(&attr);
+#  if defined(__APPLE__)
+    /* A plain pthread gets the default QoS and, on a busy host, efficiency
+     * cores. The render thread is on the frame's critical path, like the
+     * emulation (main) thread, so it asks for the same class. */
+    pthread_attr_set_qos_class_np(&attr, QOS_CLASS_USER_INTERACTIVE, 0);
+#  endif
+    int ok = pthread_create(&R.thread, &attr, render_main, NULL) == 0;
+    pthread_attr_destroy(&attr);
 #endif
     if (!ok) {
         atomic_store(&R.running, 0);