debug(timer): timer_handler 每 250 拍打印 [T]，验证定时器中断触发

sti 后卡死在 timer_msleep，静态分析 PIT/PIC/IDT/ISR/timer_handler/
msleep/sched_tick 的 IR 全部正确。加串口诊断确认 ISR 是否触发：
若串口无 [T] → 中断未触发（PIC/PIT/IDT 运行期问题）；有 [T] 仍挂 →
ticks 未推进/ISR 未返回。用后移除。

VKernel/Kernel/platform/pch/timer.vp

@@ -1,6 +1,7 @@
 import platform.pch.pic as pic
 import platform.pch.pit as pit
 import sched.sched as sched
+import drivers.serial.uart.serial as serial
 import asm
 import t, c
 
@@ -12,6 +13,7 @@ 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_last:     t.CUInt64T = 0
 
 def timer_init():
     global ticks, seconds, milliseconds, timer_running
@@ -24,7 +26,7 @@ def timer_init():
     pit.pit_init(TIMER_FREQUENCY)
 
 def timer_handler():
-    global ticks, seconds, milliseconds, timer_running
+    global ticks, seconds, milliseconds, timer_running, _dbg_last
     if not timer_running:
         pic.eoi(0)
         return
@@ -35,6 +37,10 @@ def timer_handler():
         seconds = seconds + 1
         milliseconds = milliseconds - 1000
     c.Asm("mfence", op=[t.ASM_DESCR.CLOBBER_MEMORY])
+    # [DBG] 每 250 拍打印一次，验证定时器中断是否触发（排查 sti 后挂起，用后移除）
+    if ticks - _dbg_last >= 250:
+        _dbg_last = ticks
+        serial.puts("[T]")
     sched.Scheduler.sched_tick()
     pic.eoi(0)