Function split_predicate_for_postgres 

pub fn split_predicate_for_postgres(predicate: &Predicate) -> SplitPredicate

Split a predicate into parts that can be pushed down to PostgreSQL vs evaluated post-fetch.

Pushdown-capable (translated to SQL):

• Simple column comparisons (single-step paths like name = 'value')
• JSONB path comparisons (multi-step paths like data.field = 'value')
• AND/OR/NOT combinations of pushdown-capable predicates
• IS NULL, TRUE, FALSE

Requires post-filtering (evaluated in Rust):

• Future: Ref traversals, complex expressions