[codex] Fix review logic and save performance

[zerox80]

Summary

• Treat unknown disk/OS facts as unknown instead of false HDD/Win10 upgrade failures.
• Rework assignment saves to validate against cached devices, update the cached device, and avoid a full share rescan/reload.
• Make CSV exports collision-safe and surface the configured age threshold in the overview KPI.
• Add backend/vector coverage for unknown facts, cached assignment writes, and unique export filenames.

Validation

• cargo fmt --all -- --check
• cargo clippy --all-targets -- -D warnings
• cargo test
• npm.cmd run check
• npm.cmd test
• source line limit check
• git diff --check