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

.github/workflows/ci.yml

@@ -103,6 +103,18 @@ jobs:
         run: cargo run --release -- connect --help
       - name: rustunnel generate --help
         run: cargo run --release -- generate --help
+      - name: rustunnel keygen prints a rtun3. key
+        shell: bash
+        run: |
+          KEY=$(cargo run --release -- keygen)
+          case "$KEY" in rtun3.*) ;; *) echo "ERROR: keygen output should start with rtun3., got: $KEY"; exit 1;; esac
+      - name: connect requires --target
+        shell: bash
+        run: |
+          if cargo run --release -- connect "$(cargo run --release -- keygen)" 2>&1; then
+            echo "ERROR: connect without --target should have failed"
+            exit 1
+          fi
       - name: Invalid command fails
         shell: bash
         run: |