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/reference/data-models.md

@@ -4,18 +4,13 @@
 
 ```rust
 pub struct ConnectionKey {
-    pub version: u8,          // must be 2
-    pub target: String,       // listener address
-    pub ca_cert_pem: String,
-    pub server_cert_pem: String,
-    pub server_key_pem: String,
-    pub client_cert_pem: String,
-    pub client_key_pem: String,
-    pub auth_token: String,
+    seed: [u8; 32],   // random seed from which all TLS material is derived
 }
 ```
 
-Encoded as: `rtun2.` + base64url(deflate(JSON)) — no padding.
+Encoded as: `rtun3.` + base64url(32-byte seed) — no padding (~49 chars). No
+certificates are stored in the key; both endpoints derive the same CA and
+ephemeral leaves from the seed (`src/keyderive.rs`).
 
 ## Config