fix: BLE watchdog — 좀비 연결 감지 + 강제 reconnect

Android BLE가 disconnect 콜백을 안 보내는 경우(좀비 연결) 대응:
- 마지막 RX로부터 15초간 응답 없으면 강제 disconnect → auto reconnect
- processReceivedData()에서 lastRxTimestamp 갱신
- 5초마다 watchdog 체크
- WATCHDOG_TIMEOUT 이벤트 BleDebugLogger에 기록
- 수동 disconnect 시 watchdog 해제

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/src/main/java/com/example/medilightv2android/ble/BleManager.kt

@@ -63,6 +63,9 @@ class BleManager private constructor(private val context: Context) {
     private val maxReconnectAttempts = 5
     private val reconnectDelays = longArrayOf(3000, 5000, 10000, 10000, 10000)
     private var reconnectTimer: Runnable? = null
+    private var lastRxTimestamp: Long = 0
+    private var watchdogTimer: Runnable? = null
+    private val watchdogTimeoutMs: Long = 15000
 
     val discoveredDevices = mutableStateListOf<BleDevice>()
     val isScanning = mutableStateOf(false)
@@ -220,6 +223,7 @@ class BleManager private constructor(private val context: Context) {
     fun disconnect() {
         isUserDisconnect = true
         cancelAutoReconnect()
+        stopWatchdog()
         connectionTimer?.let { handler.removeCallbacks(it) }
         connectionTimer = null
         bluetoothGatt?.disconnect()
@@ -348,6 +352,50 @@ class BleManager private constructor(private val context: Context) {
         batteryTimer = null
     }
 
+    // ── Watchdog (zombie connection detection) ──
+
+    private fun startWatchdog() {
+        stopWatchdog()
+        lastRxTimestamp = System.currentTimeMillis()
+        val check = object : Runnable {
+            override fun run() {
+                if (!isConnected.value) return
+                val silenceMs = System.currentTimeMillis() - lastRxTimestamp
+                if (silenceMs > watchdogTimeoutMs) {
+                    logw { "Watchdog: no RX for ${silenceMs}ms — forcing disconnect + reconnect" }
+                    debugLogger.error("WATCHDOG_TIMEOUT silence=${silenceMs}ms — forcing reconnect")
+                    forceDisconnectAndReconnect()
+                    return
+                }
+                handler.postDelayed(this, 5000)
+            }
+        }
+        watchdogTimer = check
+        handler.postDelayed(check, 5000)
+    }
+
+    private fun stopWatchdog() {
+        watchdogTimer?.let { handler.removeCallbacks(it) }
+        watchdogTimer = null
+    }
+
+    private fun forceDisconnectAndReconnect() {
+        stopBatteryPolling()
+        stopWatchdog()
+        isConnected.value = false
+        isServiceReady.value = false
+
+        bluetoothGatt?.let { gatt ->
+            try { gatt.disconnect(); gatt.close() } catch (_: Exception) {}
+        }
+        bluetoothGatt = null
+        txCharacteristic = null
+        rxCharacteristic = null
+
+        onUnexpectedDisconnect?.invoke()
+        scheduleAutoReconnect()
+    }
+
     // ── Auto Reconnect ──
 
     private fun scheduleAutoReconnect() {
@@ -544,6 +592,7 @@ class BleManager private constructor(private val context: Context) {
                         if (txCharacteristic != null && rxCharacteristic != null) {
                             isServiceReady.value = true
                             startBatteryPolling()
+                            startWatchdog()
                             logd { "Service ready! (CCCD written)" }
                         }
                     } else {
@@ -579,6 +628,7 @@ class BleManager private constructor(private val context: Context) {
     }
 
     private fun processReceivedData(data: ByteArray) {
+        lastRxTimestamp = System.currentTimeMillis()
         val prefix = String(data, 0, minOf(4, data.size), Charsets.US_ASCII)
         debugLogger.rx(prefix.trimEnd(':', '!'), data.size)
         logd { "Received: $prefix (${data.size} bytes)" }