feat(sdk): OTA 升级流程对齐 expo + 拆 extension 文件分层 + 秘钥本地化

主要改动：
- 新增 fetchLatestFirmware/upgradeFirmware/tryReportPendingUpgrade
  三段 OTA 接口，对齐 expo bindDeviceWithOrchestration 行为
- bind 仅做硬绑+getVersion，软绑/对账上报/拉最新固件搬到
  fetchLatestFirmware 内部串；软绑失败自动硬解回滚
- UpgradeRecord 复合主键 (sn, firmwareId) 持久化 + sn 级 in-flight 互斥
- DuooomiBleSDKDelegate 新增 didUpdateUpgradeRecords，集成方
  可观察对账上报结果（reported/outcome 状态）
- DuooomiBleSDK 主类按职责拆 5 个 extension 文件，公开 API 不变
- TargetFirmware 简化为只取必要字段 + 新增 forceUpgrade
- scanNamePrefix 改为必填 String
- demo 秘钥移至本地 DemoSecrets.swift (gitignored)
- README 重写：完整 OTA 流程示例 + 上报状态判定表

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

Sources/DuooomiBleSDK/Models/UpgradeRecord.swift

@@ -0,0 +1,44 @@
+import Foundation
+
+/// 升级对账结果。
+/// - `success`: 设备实际版本 == 目标版本，已成功上报
+/// - `versionMismatch`: 实际版本 ≠ 目标版本，已上报 failure
+/// - `nil`: 30 天 TTL 过期未上报，保留为审计轨迹
+public enum UpgradeOutcome: String, Codable, Equatable {
+    case success
+    case versionMismatch = "version_mismatch"
+}
+
+/// OTA 升级记录。复合主键 `(sn, firmwareId)`。
+/// 与 RN `bleStore.upgradeRecords` 1:1 对齐。
+public struct UpgradeRecord: Codable, Equatable {
+    public let sn: String
+    public let firmwareId: String
+    public let fromVersion: String
+    public let targetVersion: String
+    /// 触发 OTA 烧录的时间戳（毫秒，与 RN `Date.now()` 同口径）
+    public let triggeredAt: Int64
+    public let reported: Bool
+    public let reportedAt: Int64?
+    public let outcome: UpgradeOutcome?
+
+    public init(
+        sn: String,
+        firmwareId: String,
+        fromVersion: String,
+        targetVersion: String,
+        triggeredAt: Int64,
+        reported: Bool = false,
+        reportedAt: Int64? = nil,
+        outcome: UpgradeOutcome? = nil
+    ) {
+        self.sn = sn
+        self.firmwareId = firmwareId
+        self.fromVersion = fromVersion
+        self.targetVersion = targetVersion
+        self.triggeredAt = triggeredAt
+        self.reported = reported
+        self.reportedAt = reportedAt
+        self.outcome = outcome
+    }
+}