feat: add ALS-based test scope hooks foundation

[Lojhan]

Summary

• add a generic ALS-based test scope provider in dom
• register scope hooks in DOM environment setup (happy-dom and jsdom)
• export scope API from package index
• add comprehensive tests for scope lifecycle, cleanup ordering, observer behavior, concurrency isolation, and hook registration

Why

This establishes the plugin-agnostic scope infrastructure needed for per-test state isolation via poku's generic scope-hook contract.

Notes

• This is foundational wiring in dom; framework packages can now migrate from module-level global sets to per-test slots backed by this scope API.
• Compatible with poku versions that do not implement scope hooks in core, because core falls back when hooks are absent and registration on dom side is non-breaking.

Validation

• npm run lint
• npm test