feat: wire QUIC transport, JNI bridge, connect UI + add docs

- Replace raw FFI with proper `jni` crate for string marshalling
- Wire QUIC transport in engine: connect to relay, crypto handshake
  (CallOffer/CallAnswer, X25519+Ed25519), send/recv MediaPackets
- Feed received packets into jitter buffer (was previously ignored)
- Add connect screen UI with CALL button (idle state) and in-call
  controls (mute, speaker, hang up, live stats)
- Hardcode relay 172.16.81.125:4433, room "android"
- Add comprehensive docs in docs/android/:
  architecture.md (8 mermaid diagrams), build-guide.md,
  debugging.md, maintenance.md, roadmap.md

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

android/app/src/main/java/com/wzp/engine/WzpEngine.kt

@@ -32,12 +32,12 @@ class WzpEngine(private val callback: WzpCallback) {
      * Start a call.
      *
      * @param relayAddr relay server address (host:port)
-     * @param room      room identifier
-     * @param seedHex   64-char hex-encoded 32-byte identity seed
-     * @param token     authentication token
+     * @param room      room identifier (used as QUIC SNI)
+     * @param seedHex   64-char hex-encoded 32-byte identity seed (empty = random)
+     * @param token     authentication token (empty = no auth)
      * @return 0 on success, negative error code on failure
      */
-    fun startCall(relayAddr: String, room: String, seedHex: String, token: String): Int {
+    fun startCall(relayAddr: String, room: String, seedHex: String = "", token: String = ""): Int {
         check(nativeHandle != 0L) { "Engine not initialized" }
         val result = nativeStartCall(nativeHandle, relayAddr, room, seedHex, token)
         if (result == 0) {