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What & Why

This PR fixes a bug where the download verification phase never transitions to "Verified" status after successful checksum verification. Currently, users see the status stuck at "Verifying" even after verification completes successfully, making it unclear whether the process finished successfully.

User-visible impact: Download progress now properly shows the complete flow:

Downloading → Downloaded → Verifying → Verified 

Changes

Added verifying! and verified! phase methods (matching existing downloading!, downloaded!, extracting! pattern) and updated verify_download_integrity() to call verified! when verification actually succeeds