count(DISTINCT x) was a parse error ("expected token type 85, got 9").
parse_aggregate_item now consumes an optional DISTINCT inside the call and
records it on the return item. Both aggregation paths track the set of unique
values per group and emit its size for COUNT(DISTINCT): the RETURN path reuses
the existing collect_lists machinery; the WITH path gains distinct_lists/
distinct_n fields. Plain COUNT and the other aggregates are unchanged.
Unit test: COUNT(DISTINCT f.label)=1 (all same label), count(f.label)=4
(non-distinct), COUNT(DISTINCT f.name)=4 (unique names). Smoke suite adds a
count(DISTINCT) query_graph check.
Martin Vogel
869341c709