Batch 4: Download subsystem integration — replace old DownloadManager

[JusterZhu]

Batch 4: Download Subsystem Integration

Core refactor batch. Replaces the old DownloadManager/DownloadTask with the new layered download abstractions, integrates cross-version package support, and connects the DownloadProgressReporter bridge.

Tasks

1. DownloadPlan model — Create DownloadPlan record with asset list + strategy metadata
2. BuildDownloadPlan logic — Cross-version package selection, version chain building, frozen package filtering, forced update marking
3. PacketDTO cross-version fields — Add FromVersion, IsCrossVersion, MinClientVersion, SourceArchiveHash, TargetArchiveHash
4. ClientUpdateStrategy → new abstractions — Replace old DownloadManager calls with IDownloadSource → BuildDownloadPlan → IDownloadOrchestrator.ExecuteAsync
5. OSSUpdateStrategy → new abstractions — Same pattern: DownloadSource → DownloadPlan → DownloadOrchestrator
6. UpgradeUpdateStrategy pipeline — Remove old DownloadManager+DownloadAsync; read from ProcessInfo.DownloadResults
7. Delete old DownloadManager + DownloadTask — Remove after confirming no remaining references
8. DownloadProgressReporter bridge — Connect IProgress → EventManager events

Related

• Refactor plan: GeneralUpdate-refactor-plan.md