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

README.md

@@ -57,6 +57,20 @@ Use the SOCKS5 proxy from a local client:
 curl --proxy socks5h://127.0.0.1:1180 http://127.0.0.1:4181/
 ```
 
+## Connection keys
+
+Instead of certificate files you can use a short connection key, from which both
+endpoints derive identical TLS material on the spot:
+
+```sh
+rustunnel keygen                                   # prints a ~49-char rtun3. key
+rustunnel listen --listen 0.0.0.0:4180 --connection-key '<key>'   # server side
+rustunnel connect --target 127.0.0.1:4180 '<key>'                 # each client
+```
+
+The key is a random 32-byte seed; no certificates are ever shipped in it. The target
+address is passed separately on `connect`.
+
 ## Defaults and custom ports
 
 - HTTPS listener: `127.0.0.1:4180`