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/examples/demo.rb

@@ -0,0 +1,43 @@
+#!/usr/bin/env ruby
+# frozen_string_literal: true
+
+# Example: quicproquo Ruby SDK demo.
+#
+# Usage:
+#   ruby demo.rb --server 127.0.0.1:5001 --ca-cert ca.pem \
+#     --user alice --pass secret --recipient bob --message "hello"
+
+require "optparse"
+require_relative "../lib/quicproquo"
+
+options = {
+  server: "127.0.0.1:5001",
+  ca_cert: "ca.pem",
+  message: "hello from Ruby SDK!",
+}
+
+OptionParser.new do |opts|
+  opts.banner = "Usage: demo.rb [options]"
+  opts.on("--server ADDR", "Server address") { |v| options[:server] = v }
+  opts.on("--ca-cert PATH", "CA certificate") { |v| options[:ca_cert] = v }
+  opts.on("--user NAME", "Username") { |v| options[:user] = v }
+  opts.on("--pass PASSWORD", "Password") { |v| options[:pass] = v }
+  opts.on("--recipient NAME", "Recipient") { |v| options[:recipient] = v }
+  opts.on("--message TEXT", "Message") { |v| options[:message] = v }
+end.parse!
+
+QuicProQuo::Client.open(options[:server], ca_cert: options[:ca_cert]) do |client|
+  puts "Connected to #{options[:server]}"
+
+  client.login(options[:user], options[:pass])
+  puts "Logged in as #{options[:user]}"
+
+  client.send(options[:recipient], options[:message])
+  puts "Sent message to #{options[:recipient]}"
+
+  puts "Waiting for messages (5s)..."
+  messages = client.receive(timeout_ms: 5000)
+  messages.each { |msg| puts "  received: #{msg}" }
+
+  puts "Done."
+end