v0.2.0 — the first F-Droid-submittable release.

Highlights:
* In-person QR pairing + 4-word rotating verify
* Long-distance pairing via encrypted transport package
* Lost-phone recovery via printed paper or saved file
* Multi-relationship storage + in-person rekey
* Printable challenge-response grid (fallback when the other side has no phone)
* Liveness prompts for video calls
* Android screenshot blocking on sensitive screens

Alpha release: expect rough edges. Issue tracker:
https://github.com/digital-grease/signet/issues
