feat(sdk): 同步 iOS — upgradeFirmware 下发前校验电量 ≥40%，不足抛 BatteryTooLow

1:1 复刻 iOS：OTA 烧录前主动 getDeviceInfo 取实时电量，<40% 抛
DuooomiBleError.BatteryTooLow(current,required)、不发任何字节。
错误消息字符串与 iOS 逐字一致；新增 MIN_BATTERY_FOR_UPGRADE=40。
协程版线性实现（无 callback 拆分）。README 同步错误处理/升级段。

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

sdk/src/main/kotlin/com/duooomi/ble/DuooomiBleSDK.kt

@@ -452,6 +452,9 @@ class DuooomiBleSDK(context: Context, val config: DuooomiBleConfig) {
     // MARK: - High-Level APIs
 
     companion object {
+        /** OTA 烧录前置电量门槛（百分比）。低于此值拒绝下发，避免烧录中途断电变砖。 */
+        internal const val MIN_BATTERY_FOR_UPGRADE = 40
+
         private val SUPPORTED_EXTENSIONS = setOf("jpeg", "jpg", "png", "gif", "mp4", "webm", "webp")
         private val SUPPORTED_MIME_TYPES = setOf(
             "image/jpeg", "image/png", "image/gif", "image/webp",
@@ -550,6 +553,13 @@ class DuooomiBleSDK(context: Context, val config: DuooomiBleConfig) {
     /**
      * OTA 固件烧录。完成后自动落 [UpgradeRecord] 到本地（复合主键 `(sn, firmwareId)`），
      * 等待下次 [fetchLatestFirmware] 触发 [tryReportPendingUpgrade] 对账上报。
+     *
+     * 下发前会**主动拉一次新鲜 [getDeviceInfo]** 校验实时电量，低于
+     * [MIN_BATTERY_FOR_UPGRADE]（40%）直接抛 [DuooomiBleError.BatteryTooLow]，不发任何 OTA 字节。
+     *
+     * @throws DuooomiBleError.NotConnected 未连接
+     * @throws DuooomiBleError.BatteryTooLow 电量 < [MIN_BATTERY_FOR_UPGRADE]（未下发任何字节）
+     * @throws DuooomiBleError.TransferFailed 烧录阶段失败；[getDeviceInfo] 的 Timeout / 解码错误同样透传
      */
     suspend fun upgradeFirmware(
         sn: String,
@@ -559,6 +569,12 @@ class DuooomiBleSDK(context: Context, val config: DuooomiBleConfig) {
         targetVersion: String
     ) {
         ensureConnected()
+        // OTA 前置电量校验：取实时电量（不用缓存，可能过期），低于门槛拒绝下发。
+        val info = getDeviceInfo()
+        if (info.powerlevel < MIN_BATTERY_FOR_UPGRADE) {
+            BleLog.w("OTA blocked: battery ${info.powerlevel}% < $MIN_BATTERY_FOR_UPGRADE%", "Firmware")
+            throw DuooomiBleError.BatteryTooLow(current = info.powerlevel, required = MIN_BATTERY_FOR_UPGRADE)
+        }
         BleLog.i("OTA upgrade start: sn=$sn $fromVersion→$targetVersion (fw=$firmwareId)", "Firmware")
         transferFile(fileUri = fileUrl, commandType = CommandType.OTA_PACKAGE)
         BleLog.i("OTA upgrade completed", "Firmware")