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>

crates/wzp-android/Cargo.toml

@@ -25,6 +25,9 @@ thiserror = { workspace = true }
 async-trait = { workspace = true }
 anyhow = "1"
 libc = "0.2"
+jni = { version = "0.21", default-features = false }
+rand = { workspace = true }
+rustls = { version = "0.23", default-features = false, features = ["ring"] }
 
 [build-dependencies]
 cc = "1"