[client] refactor lazy detection (#4050)

This PR introduces a new inactivity package responsible for monitoring peer activity and notifying when peers become inactive.
Introduces a new Signal message type to close the peer connection after the idle timeout is reached.
Periodically checks the last activity of registered peers via a Bind interface.
Notifies via a channel when peers exceed a configurable inactivity threshold.
Default settings
DefaultInactivityThreshold is set to 15 minutes, with a minimum allowed threshold of 1 minute.

Limitations
This inactivity check does not support kernel WireGuard integration. In kernel–user space communication, the user space side will always be responsible for closing the connection.

monotime/time.go

@@ -0,0 +1,29 @@
+package monotime
+
+import (
+	"time"
+)
+
+var (
+	baseWallTime time.Time
+	baseWallNano int64
+)
+
+func init() {
+	baseWallTime = time.Now()
+	baseWallNano = baseWallTime.UnixNano()
+}
+
+// Now returns the current time as Unix nanoseconds (int64).
+// It uses monotonic time measurement from the base time to ensure
+// the returned value increases monotonically and is not affected
+// by system clock adjustments.
+//
+// Performance optimization: By capturing the base wall time once at startup
+// and using time.Since() for elapsed calculation, this avoids repeated
+// time.Now() calls and leverages Go's internal monotonic clock for
+// efficient duration measurement.
+func Now() int64 {
+	elapsed := time.Since(baseWallTime)
+	return baseWallNano + int64(elapsed)
+}