feat(packages): add shared contracts and UI primitive foundations

[d9ff1ck]

Summary

• Added @simplix/contracts for shared TypeScript contracts: equality, readable/writable APIs, snapshots, subscriptions, and lifecycle primitives.
• Refactored @simplix/core around shared contracts, lifecycle cleanup registry, key/token identity primitives, mutable state snapshots, and stateful machine behavior.
• Added headless checkbox state/controller logic and wired it into React DOM through a checkbox adapter/component.
• Renamed DOM package to @simplix/vanilla-dom, cleaned event binder internals, and added styling dataset types.
• Added React DOM button/checkbox primitives and tightened polymorphic/layout typings.
• Added package-scoped CI checks, scoped .gitignore files, workspace config cleanup, and a Vite React playground shell.

Validation

• Built packages in dependency order through @simplix/playground.
• @simplix/core typecheck, tests, and build pass.