Deduplicate shared listener endpoints

[samuel-williams-shopify]

Summary

• publish a listener reported by multiple supervised workers as one EDS endpoint
• keep the endpoint healthy while any reporting worker is healthy
• document standard Falcon server and cluster listener registration

This supports the shared Falcon::Listener lifecycle introduced by socketry/falcon#356.

Verification

• bundle exec bake test
• bundle exec rubocop
• bundle exec bake decode:index:coverage lib