Valtio proxy-based state

Leverage Valtio's proxy magic for state. Usage: 1. proxy() creates mutable state. 2. Direct property mutations. 3. snapshot() for immutable reads. 4. useSnapshot hook in React. 5. subscribe() for listeners. 6. Nested objects auto-tracked. 7. Derive computed values. 8. Works outside React. Simple mental model: mutate state directly, components auto-update. Use with class instances for OOP patterns.