PackageType enum migration + chain/full fallback mechanism

[JusterZhu]

Motivation

PackageType.Chain = 0 collides with int default (0), making uninitialized fields silently behave as Chain. Need to migrate Chain→1, Full→2 (unchanged), add Unspecified=0 as safe default.

Changes

• PackageType.cs: Chain = 0 \→ Chain = 1, add Unspecified = 0
• DownloadPlanBuilder: Remove CVP matching logic; chain total >= 80% full → use full directly; attach FallbackFull* info for chain→full retry
• DownloadAsset: Add PackageType, FallbackFullName/Url/Hash, FallbackFulls
• AbstractStrategy: On chain failure, retry with matching fallback full
• ClientStrategy: Propagate fallback info; download fallback fulls alongside chain
• CompressMiddleware/Platform Strategies: Full packages skip PatchMiddleware, decompress directly to install path
• Cleanup: Remove all CVP concepts (matchingCvp, IsCrossVersion, FromVersion, SourceArchiveHash, TargetArchiveHash)

Verification

• All projects build successfully
• Chain → full fallback triggers when chain package fails
• Size-based switch triggers when chain total >= 80% of full