Revert incomplete resumable downloads implementation

[Copilot]

The resumable downloads implementation introduced test failures and security alerts. The implementation attempted to add resume logic at the store layer without proper integration with the HTTP fetcher layer.

Changes

• Reverted store-level resume logic - Removed incomplete file persistence, GetIncompleteSize() method, and hash verification for resumed downloads from blobs.go
• Removed HTTP Range request handling - Removed context-based offset propagation and partial content handling from fetcher.go and image.go
• Removed progress offset tracking - Deleted NewReaderWithOffset() function from reader.go
• Cleaned up client integration - Removed resume offset detection and context injection from client.go

Why the revert

The implementation had architectural issues:

1. Store layer appended to incomplete files but received full-content readers from HTTP layer
2. Hash verification ran on concatenated partial+full data instead of complete file
3. Range request context wasn't propagated through the layer stack
4. Test suite expected incomplete files to be cleaned up on error

Net result: -242 lines, all tests passing, zero security alerts.

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