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/index.md

@@ -9,7 +9,7 @@ The tool is designed for local labs, development environments, and controlled te
 - **Listener** (`rustunnel listen`) — binds an HTTPS server that accepts mTLS connections from connectors, validates an auth token, then upgrades the connection to a persistent binary-framed tunnel.
 - **Connector** (`rustunnel connect`) — connects to the listener over HTTPS with mTLS, authenticates, then exposes a local SOCKS5 proxy that forwards traffic through the tunnel.
 - **Credential generation** (`rustunnel generate`) — creates a self-signed CA, server certificate, client certificate, and auth token for a local session.
-- **Connection keys** (`rustunnel keygen`) — bundles all credential material into a single compressed base64url string that can be copy-pasted between machines.
+- **Connection keys** (`rustunnel keygen`) — a short ~49-char seed from which both endpoints derive identical TLS material and which can be copy-pasted between machines.
 
 ## Quick links