Add Falcon cluster service environment

[samuel-williams-shopify]

Summary

• Add Falcon::Environment::Cluster and Falcon::Service::Cluster for workers that bind independently inside their execution context.
• Describe each post-bind worker resource as an immutable Falcon::Service::Cluster::Listener.
• Expose the listener name, scheme, supported wire-protocol names, bound endpoint, and every concrete socket address.
• Invoke prepare_worker!(instance, listener:) after binding and before running the server.
• Ensure each worker closes its bound endpoint.
• Add a two-worker Unix-domain-socket cluster example using process and thread identity in each path.

The evaluator remains lazy configuration state; live bound sockets are passed explicitly through the worker lifecycle hook. Integrations can serialize the listener's scheme and protocol capabilities into their own discovery model without depending on Async::HTTP implementation constants.

Released dependencies

• async-http v0.97.0
• async-service-supervisor v0.18.0

Companion PRs

• Support grouped Unix endpoint addresses async-grpc-xds#1
• Register Falcon cluster endpoints with Envoy async-service-supervisor-envoy#1

Testing

• bundle exec sus test/falcon/service/cluster.rb test/falcon/environment/cluster.rb (4 tests, 30 assertions)
• bundle exec rubocop
• bundle exec bake decode:index:coverage lib (201/201 definitions)