fix(hub): verify download integrity and atomically promote models (fixes #113)

[BerryUIKI]

Closes #113.

Changes

• Validated \Content-Range\ headers upon resume to ensure server resumed from \existing_bytes; if incompatible or HTTP 200 returned, cleanly reset and redownload from byte 0.
• Added pre-promotion expected size check against declared \model.size_bytes\ or server total.
• Added SHA-256 integrity verification comparing downloaded chunked hash against \model.sha256.
• Replaced destructive \ arget_path.unlink()\ with atomic \Path.replace()\ on the destination, preserving any prior file if verification or promotion fails.
• Added regression tests covering size mismatch rejection, SHA-256 mismatch rejection, and atomic replacement preservation.