fix: prevent BLE freeze on NIRS streaming stop

Clear mcj callback immediately on stop to prevent stale responses from
triggering new polling chains. Delay battery polling resume by 500ms to
avoid command collision with in-flight mcj response. Add sendNirsPowerOff
for future use.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

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

@@ -287,6 +287,10 @@ class BleManager private constructor(private val context: Context) {
         sendRaw(CRC16.buildCommand("mta", intArrayOf(1)))
     }
 
+    fun sendNirsPowerOff() {
+        sendRaw(CRC16.buildCommand("mta", intArrayOf(0)))
+    }
+
     fun sendMqqQuery() {
         sendRaw(CRC16.buildCommand("mqq"))
     }