Reactive Signal, Computed & Effect Engine from Scratch

### Problem Overview Modern frontend frameworks (Angular Signals, SolidJS, Vue Reactivity, Preact Signals) rely on fine-grained reactivity. Your goal is to build the core reactive primitives from scratch: `signal()`, `computed()`, and `effect()`. ### Functional Requirements 1. **`createSignal<T>(initialValue: T): [() => T, (nextValue: T | ((prev: T) => T)) => void]`**: - Returns a getter that automatically registers the active running effect as a subscriber. - Returns a setter that triggers registered subscribers when the value changes (`Object.is` equality check). 2. **`createComputed<T>(fn: () => T): () => T`**: - Lazy evaluation: only recalculates when dependencies change. - Memoizes the calculated value. 3. **`createEffect(fn: () => void | (() => void)): () => void`**: - Automatically tracks all signals accessed during execution. - Re-runs whenever any dependency updates. - Supports cleanup functions returned from previous effect executions. 4. **`batch(fn: () => void): void`**: - Batches multiple signal updates so effects only re-execute once at the end of the batch. ### Code Smell Guardrails - **Zombie Subscriptions & Memory Leaks**: When an effect branches into different conditional logic (`if (cond) sigA() else sigB()`), unaccessed signals must be cleaned up from the dependency list! - **Infinite Loops**: Circular dependencies must be detected or handled gracefully.