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/getting-started.md

@@ -76,26 +76,27 @@ curl --proxy socks5h://127.0.0.1:1180 http://127.0.0.1:4181/
 
 ## Using a connection key
 
-Instead of passing individual certificate paths, you can generate a single connection key:
+Instead of passing individual certificate paths, you can use a short connection key
+from which both ends derive identical credentials:
 
 ```sh
-./target/release/rustunnel keygen --target 127.0.0.1:4180
+./target/release/rustunnel keygen
 ```
 
-Then start the listener with the key (it auto-generates credentials):
+Start the listener with the key:
 
 ```sh
-./target/release/rustunnel listen --connection-key <key>
+./target/release/rustunnel listen --listen 0.0.0.0:4180 --connection-key <key>
 ```
 
-And connect with the same key:
+Connect with the same key, passing the listener address explicitly:
 
 ```sh
-./target/release/rustunnel connect <key>
+./target/release/rustunnel connect --target 127.0.0.1:4180 <key>
 ```
 
 ## Defaults
 
 - Listener bind address: `0.0.0.0:4180`
-- Connector target: required (or taken from connection key)
+- Connector target: required (not stored in the key)
 - Connector SOCKS5 proxy: `127.0.0.1:1180`