Connection keys are now a ~49-char seed (rtun3.) instead of a bundled
~1740-char certificate blob. Both endpoints deterministically derive an
identical Ed25519 CA from the seed and mint ephemeral server/client leaves
at startup (keyderive.rs); the app-layer auth token is derived from the seed.
Target is passed separately on connect (resocks-style).
- connkey.rs: rtun3 seed parse/format
- keyderive.rs: CA/server/client/token derivation
- keygen takes no args; connect requires --target
- remove miniz_oxide; TLS layer unchanged
- add determinism + key-based e2e + wrong-seed-rejected tests
- update wiki, README, design spec, CI smoke
