Expose per-worker utilization snapshots

[samuel-williams-shopify]

Summary

• expose immutable per-worker utilization snapshots keyed by supervisor worker ID
• retain supervisor state alongside each utilization reading
• build existing service-level aggregation from the per-worker snapshot

This provides the narrow generic boundary needed for Envoy ORCA reporting without exposing shared-memory allocator internals.

Verification

• bundle exec bake test
• bundle exec rubocop lib/async/service/supervisor/utilization_monitor.rb test/async/service/utilization_monitor.rb
• bundle exec bake decode:index:coverage lib