Phase 11: Add integrity verification, checksums and atomic finalization

[ashishsinghbora]

Goal

Guarantee that a completed file is exactly what the server delivered.

Scope

• Add SHA-256 verification support when an expected digest is available.
• Provide optional post-download hashing when no remote checksum exists.
• Track per-chunk completion safely.
• Validate final file size before marking complete.
• Atomically rename completed .part files.
• Detect truncated or oversized responses.
• Add configurable retention/cleanup rules for partial files.

Acceptance criteria

• Corrupted test data is detected.
• A crash during finalization cannot produce a false completed state.
• File size and checksum checks are tested.