Function normalise_root_constraint_deep 

pub fn normalise_root_constraint_deep(
    constraint: &Expression,
) -> Option<Expression>

Deep-normalises a single top-level constraint, or returns None if it is already normal.

Callers that know which root child changed should prefer this over rebuilding the whole root: the root hook runs after every rewrite, so touching siblings there costs O(model size) per rewrite.