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.
• Add the immutable Falcon::Listener runtime abstraction for both regular servers and cluster workers.
• Expose each listener's name, scheme, supported wire-protocol names, bound endpoint, and every concrete socket address.
• Use one shared listener for Falcon::Service::Server and one worker-owned listener per Falcon::Service::Cluster worker.
• Invoke prepare_worker!(instance, listener:) after binding and before running either server mode.
• Keep listener ownership explicit so regular servers close during service shutdown and cluster workers close during worker shutdown.
• Add a Docker Compose example running a two-worker cluster on ephemeral TCP ports behind Envoy using xDS/EDS.

The evaluator remains lazy configuration state; live bound sockets are passed explicitly through the worker lifecycle hook. Integrations 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
• async-grpc-xds v0.1.0
• async-service-supervisor-envoy v0.0.1

Companion PRs

• Support grouped Unix endpoint addresses async-grpc-xds#1 (merged)
• Register Falcon cluster endpoints with Envoy async-service-supervisor-envoy#1 (merged)
• Deduplicate shared listener endpoints async-service-supervisor-envoy#3 (shared-listener endpoint deduplication)

Testing

• bundle exec bake test (109 tests, 475 assertions)
• bundle exec rubocop (198 files, no offenses)
• bundle exec bake decode:index:coverage lib (204/204 definitions)
• Cluster integration test confirms both independently bound worker ports remain reachable.
• Docker Compose was previously verified end-to-end through Envoy; the direct cluster lifecycle test was rerun after the listener refactor.