Add shared memory segment store

[samuel-williams-shopify]

Summary

• add Async::Utilization::SegmentStore for storing, reusing, growing, and reading schema-backed shared memory segments
• expose path-based resource acquisition through SegmentStore.open
• keep initialize focused on acquired resources and store-internal state
• validate configuration before replacement and clean up partially acquired resources on failure
• support block-form opening with automatic close
• document coordinated store/observer usage and add an unreleased note

This moves the shared-memory ownership and reader-side primitive next to Async::Utilization::Observer, allowing supervisor integrations to depend on a reusable utilization abstraction.

Verification

• bundle exec bake test (50 tests, 139 assertions)
• bundle exec rubocop (236 files, no offenses)
• bundle exec bake decode:index:coverage lib (39/39 public definitions)
• COVERAGE=PartialSummary bundle exec bake test (229/229 lines, 100% coverage)

MRI 3.3, 3.4, 4.0, and head checks previously passed on Linux/macOS. The allowed-to-fail JRuby and TruffleRuby jobs retain the existing IO::Buffer mapping failures also present on main.