bug: SELECT DISTINCT returns incomplete results

[mjm918]

Bug Description

The DISTINCT keyword does not correctly deduplicate results. It returns fewer rows than expected.

Severity

Critical

Failing Tests

• select_distinct

Reproduction

CREATE TABLE items (id INTEGER PRIMARY KEY, category TEXT);
INSERT INTO items VALUES (1, 'A'), (2, 'A'), (3, 'B'), (4, 'B'), (5, 'C');

-- Should return 3 distinct categories
SELECT DISTINCT category FROM items;

Expected Behavior

Returns 3 rows: A, B, C

Actual Behavior

Returns only 1 row

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Split from #13