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/lib/quicproquo.rb

@@ -0,0 +1,23 @@
+# frozen_string_literal: true
+
+require_relative "quicproquo/ffi_bindings"
+require_relative "quicproquo/client"
+require_relative "quicproquo/errors"
+require_relative "quicproquo/version"
+
+# Ruby SDK for the quicproquo E2E encrypted messenger.
+#
+# Two usage patterns:
+#
+#   # 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")
+#   end
+#
+#   # Manual lifecycle
+#   client = QuicProQuo::Client.new("127.0.0.1:5001", ca_cert: "ca.pem")
+#   client.login("alice", "secret")
+#   client.disconnect
+module QuicProQuo
+end