v0.7.1 — resilient vulnerability polling

Fixed\n\n- Preserve confirmed vulnerability matches and pending DSH tasks when OSV is temporarily unavailable.\n- Never emit a false resolved event from a failed OSV check.\n- Expose sourceErrors in CLI/JSON and make one-shot checks fail closed on source errors.\n- Add a deterministic source-outage showcase.\n\n## Verification\n\n- 63 automated tests passing\n- real DSH headless handoff passing\n- live OSV/npm/DSH proof passing