Rebuild container child lifecycle

[samuel-williams-shopify]

Summary

• Add reusable child primitives with explicit threaded/forked child implementations and callable child types.
• Rework Generic/Group around child-owned state, event publication, restart/stop handling, and public factory compatibility.
• Restore Controller behavior with the current async-signals API and add shared public container specs for Forked, Threaded, and Hybrid.

Tests

• bundle exec sus