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/quicproquo.gemspec

@@ -0,0 +1,20 @@
+Gem::Specification.new do |s|
+  s.name        = "quicproquo"
+  s.version     = "0.1.0"
+  s.summary     = "Ruby SDK for quicproquo E2E encrypted messenger"
+  s.description = "Ruby FFI bindings to libquicproquo_ffi for the quicproquo " \
+                  "end-to-end encrypted messaging system."
+  s.authors     = ["quicproquo contributors"]
+  s.license     = "MIT"
+  s.homepage    = "https://github.com/nicholasgasior/quicproquo"
+
+  s.required_ruby_version = ">= 3.1"
+
+  s.files = Dir["lib/**/*.rb", "README.md", "LICENSE"]
+
+  s.add_dependency "ffi", "~> 1.16"
+  s.add_dependency "json", "~> 2.7"
+
+  s.add_development_dependency "rspec", "~> 3.13"
+  s.add_development_dependency "rubocop", "~> 1.60"
+end