Implements a flexible layout widget (row/column) with configurable axis, cross-axis alignment, main-axis alignment, and flex factors for distributing space among children. Includes intrinsic size computation and baseline alignment. This is a clean, self-contained layout component that could be extracted for use in any Rust UI project requiring a flexbox-like layout.

Defines the `Lens` trait and a suite of combinators (`then`, `map`, `index`, `in_arc`, `not`, `deref`, `as_ref`) for type-safe, composable access to subfields of immutable data structures. This pattern is reusable in any Rust application that needs to project and update nested state without boilerplate, and demonstrates a functional programming approach to state management.

Provides a reusable `ScrollComponent` that handles scrollbar rendering, hover/held state management, and fade-in/out animations. It can be embedded into any scrollable widget to add scrollbar functionality with minimal coupling. The component uses timer-based animation for scrollbar fading and supports both vertical and horizontal scrolling.

Implements the `Controller` trait, which allows adding behavior to a widget without subclassing or modifying the widget itself. Controllers wrap a child widget and can intercept events, lifecycle calls, and updates. This is a clean composition pattern that can be reused in any widget system to separate concerns and add cross-cutting behavior (e.g., logging, validation, animation).

Defines `BoxConstraints` with min/max width/height, aspect ratio enforcement, and loosen/tighten operations. Inspired by Flutter's layout system, this provides a robust foundation for constraint-based layout that can be reused in any 2D layout engine. The implementation is self-contained and well-documented.