feat(sdk): add Java and Ruby SDK wrappers over C FFI

Java SDK: JNI bindings to libquicproquo_ffi with QpqClient class,
Gradle build, and exception hierarchy matching Kotlin SDK.

Ruby SDK: FFI gem wrapping libquicproquo_ffi with Client class,
block-form auto-disconnect, gemspec for RubyGems publishing,
and example script.

sdks/java/README.md

@@ -0,0 +1,50 @@
+# QuicProQuo Java SDK
+
+Java wrapper over `libquicproquo_ffi` via JNI for JVM and Android.
+
+## Prerequisites
+
+- JDK 17+
+- `libquicproquo_ffi` built for the target platform
+- JNI bridge compiled (shared with Kotlin SDK: `../kotlin/jni/`)
+
+## Building
+
+```sh
+# Build Rust FFI library
+cargo build --release -p quicproquo-ffi
+
+# Build Java SDK
+./gradlew build
+```
+
+## Usage
+
+```java
+import dev.quicproquo.QpqClient;
+
+try (QpqClient client = new QpqClient("127.0.0.1:5001", "ca.pem")) {
+    client.login("alice", "secret");
+    client.send("bob", "hello".getBytes());
+
+    var messages = client.receive(5000);
+    messages.forEach(msg -> System.out.println("Received: " + msg));
+}
+```
+
+## API
+
+| Method | Description |
+|---|---|
+| `new QpqClient(server, caCertPath)` | Connect to server |
+| `client.login(username, password)` | OPAQUE authentication |
+| `client.send(recipient, message)` | Send message by username |
+| `client.receive(timeoutMs)` | Receive pending messages |
+| `client.close()` / `client.disconnect()` | Disconnect |
+| `client.isConnected()` | Connection status |
+
+## Structure
+
+- `src/main/java/dev/quicproquo/QpqClient.java` -- High-level client
+- `src/main/java/dev/quicproquo/NativeBridge.java` -- JNI declarations
+- JNI C bridge shared with Kotlin SDK at `../kotlin/jni/`