debug(timer): timer_handler 一次性 H + msleep 自旋 s，判定 ISR 是否被调用

apic.eoi 修复后仍挂在 sti done（无 msleep done）。加两处确定性诊断：
- timer_handler 首次进入打印 'H'：ISR 被调用则出现
- msleep 每 1M 次迭代打印 's'：hlt 唤醒（中断触发）但 ticks 不推进则出现
判定：
- 无 H → 定时中断未到达 timer_handler（IDT/PIC/APIC 路由）
- 有 H → ISR 已调用，问题在 handler/ticks 推进
用后移除。

VKernel/Kernel/platform/pch/timer.vp

@@ -2,6 +2,7 @@ import platform.pch.pic as pic
 import platform.pch.pit as pit
 import sched.sched as sched
 import drivers.core.cpu.apic as apic
+import drivers.serial.uart.serial as serial
 import asm
 import t, c
 
@@ -13,6 +14,7 @@ ticks:         t.CStatic | t.CVolatile | t.CUInt64T = 0
 seconds:       t.CStatic | t.CVolatile | t.CUInt32T = 0
 milliseconds:  t.CStatic | t.CVolatile | t.CUInt32T = 0
 timer_running: t.CStatic | t.CVolatile | bool       = False
+_dbg_once:     t.CInt = 0
 
 def timer_init():
     global ticks, seconds, milliseconds, timer_running
@@ -25,7 +27,11 @@ def timer_init():
     pit.pit_init(TIMER_FREQUENCY)
 
 def timer_handler():
-    global ticks, seconds, milliseconds, timer_running
+    global ticks, seconds, milliseconds, timer_running, _dbg_once
+    # [DBG] 一次性打印：'H' 出现说明定时 ISR 被调用（用后移除）
+    if _dbg_once == 0:
+        _dbg_once = 1
+        serial.puts("H")
     if not timer_running:
         pic.eoi(0)
         apic.eoi()
@@ -95,7 +101,13 @@ def timer_get_time(out_seconds: t.CUInt32T | t.CPtr, out_milliseconds: t.CUInt32
 
 def timer_msleep(ms: t.CUInt32T):
     target: t.CUInt64T = ticks + (ms + MILLISECONDS_PER_TICK - 1) / MILLISECONDS_PER_TICK
+    _spin: t.CUInt64T = 0
     while ticks < target:
+        # [DBG] 自旋计数：每 1M 次迭代打印 s——hlt 能唤醒说明中断触发
+        # （用后移除）
+        _spin += 1
+        if (_spin & 0xFFFFF) == 0:
+            serial.puts("s")
         asm.sti()
         asm.hlt()
         sched.Scheduler._yield()