Function normalize

pub fn normalize(sel: &Selection) -> NormalizedSelection

Normalizes a Selection toward a canonical form for structural comparison.

This rewrites the selection to eliminate redundant subtrees and bring structurally similar selections into a common representation. The result is suitable for comparison, hashing, and deduplication (e.g., in [RoutingFrameKey]).

Normalization preserves semantics but may alter syntactic structure. It is designed to improve over time as additional rewrites (e.g., flattening, simplification) are introduced.