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/ruby/README.md

@@ -0,0 +1,82 @@
+# QuicProQuo Ruby SDK
+
+Ruby FFI gem wrapping `libquicproquo_ffi` for the quicproquo E2E encrypted messenger.
+
+## Prerequisites
+
+- Ruby 3.1+
+- `libquicproquo_ffi` built for the target platform
+
+## Installation
+
+```sh
+gem install quicproquo
+```
+
+Or add to your Gemfile:
+
+```ruby
+gem "quicproquo"
+```
+
+## Building the Native Library
+
+```sh
+cargo build --release -p quicproquo-ffi
+```
+
+Set `QPQ_LIB_PATH` if the library is not in the default search path.
+
+## Usage
+
+```ruby
+require "quicproquo"
+
+# Block form (auto-disconnect)
+QuicProQuo::Client.open("127.0.0.1:5001", ca_cert: "ca.pem") do |client|
+  client.login("alice", "secret")
+  client.send("bob", "hello from Ruby!")
+
+  messages = client.receive(timeout_ms: 5000)
+  messages.each { |msg| puts msg }
+end
+
+# Manual lifecycle
+client = QuicProQuo::Client.new("127.0.0.1:5001", ca_cert: "ca.pem")
+client.login("alice", "secret")
+client.send("bob", "hello")
+client.disconnect
+```
+
+## API
+
+| Method | Description |
+|---|---|
+| `Client.new(server, ca_cert:, server_name:)` | Connect to server |
+| `Client.open(server, **opts) { \|c\| ... }` | Connect with auto-disconnect |
+| `client.login(username, password)` | OPAQUE authentication |
+| `client.send(recipient, message)` | Send message by username |
+| `client.receive(timeout_ms: 5000)` | Receive pending messages |
+| `client.disconnect` | Disconnect |
+| `client.connected?` | Connection status |
+
+## Error Handling
+
+```ruby
+begin
+  client.login("alice", "wrong")
+rescue QuicProQuo::AuthError => e
+  puts "Auth failed: #{e.message}"
+rescue QuicProQuo::TimeoutError => e
+  puts "Timeout: #{e.message}"
+rescue QuicProQuo::Error => e
+  puts "Error: #{e.message}"
+end
+```
+
+## Structure
+
+- `lib/quicproquo/client.rb` -- High-level client
+- `lib/quicproquo/ffi_bindings.rb` -- FFI function declarations
+- `lib/quicproquo/errors.rb` -- Exception classes
+- `examples/demo.rb` -- Usage example