Track following counts in local history

[Navesz]

Summary

• preserve following counts in local snapshots, recent orbits, timelines and CSV exports
• include following deltas alongside the existing public signals
• migrate history and backup formats to version 3
• accept v1/v2 data without inventing unavailable following counts
• document the expanded six-signal timeline

Validation

• npm run lint
• npm test (140 passing)
• npm audit --omit=dev (0 vulnerabilities)