Sub Issue 4b: Download orchestrator + status reporting + DownloadTask integration

## Changes
1. IDownloadOrchestrator — batch download coordinator with parallel/concurrency control
2. IUpdateReporter — update status reporting (UpdateStarted/DownloadCompleted/UpdateApplied/UpdateFailed/AppStarted)
3. Update DownloadTask to use HttpDownloadExecutor + DefaultRetryPolicy
4. Update DownloadManager to use IDownloadOrchestrator

Part of Sub Issue 4 of GeneralUpdate v2 refactor plan.