feat: replace connection keys with short seed-derived rtun3 keys

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

droid-wiki/overview/glossary.md

@@ -1,6 +1,6 @@
 # Glossary
 
-**Connection key** — a base64url-encoded, DEFLATE-compressed JSON blob (prefix `rtun2.`) that bundles the CA certificate, server certificate, server key, client certificate, client key, target address, auth token, and version. Generated by `rustunnel keygen` and consumed by `rustunnel listen` and `rustunnel connect`.
+**Connection key** — a short (~49-char) base64url-encoded 32-byte seed (prefix `rtun3.`). Both endpoints derive an identical CA and ephemeral leaf certificates from the seed, so no certificates are ever shipped. Generated by `rustunnel keygen` and consumed by `rustunnel listen` and `rustunnel connect`.
 
 **Connector** — the `rustunnel connect` side. Establishes an outbound HTTPS+mTLS connection to the listener, authenticates, then exposes a local SOCKS5 proxy for local applications.