//! Layer 4: Evaluation
//!
//! # Purpose
//!
//! This layer calculates high-level statistical metrics based on the smoothing results:
//! - Cross-validation for parameter selection
//! - Diagnostic metrics for fit quality
//! - Confidence and prediction intervals
//!
//! # Architecture
//!
//! ```text
//! Layer 7: API
//! ↓
//! Layer 6: Adapters
//! ↓
//! Layer 5: Engine
//! ↓
//! Layer 4: Evaluation ← You are here
//! ↓
//! Layer 3: Algorithms
//! ↓
//! Layer 2: Math
//! ↓
//! Layer 1: Primitives
//! ```
/// Cross-validation for bandwidth selection.
/// Diagnostic metrics for fit quality assessment.
/// Confidence and prediction interval computation.