Sub Issue 4a: Download abstractions + HttpDownloadExecutor + retry policy

[JusterZhu]

Scope

First phase of download subsystem refactoring.

Changes

1. Create download abstractions: IDownloadSource, IDownloadExecutor, IDownloadOrchestrator, IDownloadPolicy
2. Data models: DownloadAsset, DownloadProgress, DownloadResult, DownloadReport
3. HttpDownloadExecutor — HTTP Range download with resume support
4. DefaultRetryPolicy — exponential backoff (1s/2s/4s, max 3 retries)
5. Update DownloadTask to use new executor

See GeneralUpdate-refactor-plan.md Sub Issue 4 for full design.

Part of Sub Issue 4 of GeneralUpdate v2 refactor plan.