Simplify container supervision using Async

[samuel-williams-shopify]

Summary

• Replace the internal Group fiber/IO.select loop with an Async-backed supervision registry.
• Move Generic#spawn supervision onto Async tasks while preserving the public container API.
• Fork child processes from a short-lived thread to avoid inheriting parent scheduler state.
• Add compatibility coverage for manual container usage with Async { container.wait }.
• Rewrite Group tests around behavior instead of the old IO => Fiber internals.

Notes

This is a first simplification pass. It keeps the existing notification-pipe-driven transition to child.wait for compatibility, but the next internal simplification can make Process.wait/Thread#join the source of truth and run notification reading plus health checks alongside it.

Tests

• bundle exec sus (170 passed out of 170 total)