perf(scene3d): cap render loop at ~60 FPS to keep UI responsive

Apps/Scene3D/main.vp

@@ -3,8 +3,19 @@ import vpsdk.process as process
 import vpsdk.vpui as vpui
 import string
 from stdint import *
+import vpsdk.syscall as syscall
 import gfx
 
+class _Timespec:
+    tv_sec: int64
+    tv_nsec: int64
+
+def _sleep_ms(ms: int):
+    ts: _Timespec
+    ts.tv_sec = int64(ms / 1000)
+    ts.tv_nsec = int64((ms % 1000) * 1000000)
+    syscall.nanosleep(&ts, None)
+
 define SCENE_W = 640
 define SCENE_H = 360
 
@@ -35,7 +46,7 @@ def _render_thread(arg: rawptr):
         _time_val += float(0.02)
         if _time_val >= TIME_WRAP: _time_val = _time_val - TIME_WRAP
         root.update()
-        process.yield_cpu()
+        _sleep_ms(16)
     _render_done = 1
     # 窗口已被 explorer 回收：显式 THREAD_EXIT，绝不 return —— 用户线程的 return 会落到
     # 出口跳板（旧调用号 34，而 THREAD_EXIT=60）⇒ Ring3 `jmp $` 空转、全机不再调度。