split stmts partitions into (@property rules, other statements).

dedup props deduplicates @property rules by name, keeping the first of each name. CSS Properties and Values API 1 sec. 2 gives the last registration for a name, which is what cascade's canonicaliser keeps; nothing rides on the difference here because tw's duplicates all come from one Var and are identical.

initial_values stmts extracts (name, initial-value declaration) pairs from @property rules, in order.

sort_by_order f pairs sorts (name, initial) pairs using a provided name->order function.