SerializableSplit
Wire-portable Split over a dense F64VectorView context. Sealed + serializable so tree snapshots round-trip cleanly through kotlinx.serialization. Built-in implementations: ThresholdSplit (numeric x[i] <= t) and ExprSplit (wrapping a BoolExpr); callers needing custom predicates compose them as BoolExpr AST nodes and wrap in ExprSplit.