Use async-utilization segment store

[samuel-williams-shopify]

Summary

• replace the nested UtilizationMonitor::SegmentAllocator implementation with Async::Utilization::SegmentStore
• preserve restart-safe backing-file replacement by passing replace: true
• retain the old nested SegmentAllocator constant as a deprecated compatibility alias
• move the store-specific restart test to async-utilization while retaining the supervisor end-to-end resize regression
• require the released async-utilization ~> 0.5

Dependency

Uses async-utilization 0.5.0, which contains the new Async::Utilization::SegmentStore API.

Verification

Validated against the released async-utilization 0.5.0 without local load-path overrides:

• sus (68 tests, 212 assertions)
• rubocop (61 files, no offenses)
• documentation coverage (91/91 public definitions)
• test coverage generated successfully (89.61%)
• runtime confirmed segment_store.rb was loaded from the installed async-utilization-0.5.0 gem
• git diff --check

The refreshed dependency-sensitive CI jobs pass. The repository coverage threshold and TruffleRuby jobs also fail on the current main branch and are unrelated to this extraction.