fix: monitor worker non-blocking + test fixes

- core/monitor.py: add fast metrics script (0.3s vs 1.4s sleep),
  collect() now defaults to fast=True for responsive UI
- ui/workers.py: MonitorWorker changed from loop mode to single-shot
  (QTimer re-arms next cycle on finished signal, avoids paramiko
  blocking in infinite loop)
- ui/widgets.py: remove debug print from _kick_one_sample
- test_monitor_nonblock.py: fix sample counting by wrapping _on_sample
  instead of disconnecting signals (old approach missed new workers)
- test_process_monitor.py: assertions account for MAX_RENDER_ROWS=200,
  fix MEM% column index (4 not 3) in sort test

ui/workers.py

@@ -50,8 +50,8 @@ class CommandWorker(QThread):
         self.finished_with.emit(code, out, err)
 
 
-class MonitorWorker(QThread):
-    """后台采集系统指标；循环模式"""
+class _SystemMonitorWorker(QThread):
+    """单次采集 + QTimer 周期触发（避免 run() 死循环 + paramiko 阻塞问题）"""
     sample_ready = pyqtSignal(dict)
     error = pyqtSignal(str)
 
@@ -65,16 +65,18 @@ class MonitorWorker(QThread):
         self._stop = True
 
     def run(self):
-        while not self._stop:
-            try:
-                m = SystemMonitor.collect(self.conn)
-                self.sample_ready.emit(m)
-            except Exception as e:
-                self.error.emit(str(e))
-            for _ in range(self.interval * 10):
-                if self._stop:
-                    return
-                time.sleep(0.1)
+        """单次采集一帧；由主线程用 QTimer 调度下一次"""
+        if self._stop:
+            return
+        try:
+            m = SystemMonitor.collect(self.conn)
+            self.sample_ready.emit(m)
+        except Exception as e:
+            self.error.emit(str(e))
+
+
+# 保留旧类名做兼容（一些其他地方可能引用了 MonitorWorker）
+MonitorWorker = _SystemMonitorWorker
 
 
 class UploadWorker(QThread):