SQL Optimization: Covering Index for SELECT-Only Queries

> This problem is paper-only - see explanation for the canonical SQL reasoning. A **covering index** includes every column the query touches (SELECT, WHERE, ORDER BY, GROUP BY) so the engine can answer from the index alone, skipping the heap / clustered table read. The auto-grader validates which columns must be in the index. Given a query parsed into `{ where: string[], orderBy: string[], select: string[] }`, return the **minimum** set of columns needed for a covering index, with this layout: 1. WHERE columns first (deduped, original order), 2. ORDER BY columns next (deduped against WHERE, original order), 3. Remaining SELECT columns last (deduped against the previous two, in input order). These can go in INCLUDE clause for engines that support it (Postgres / SQL Server). **For the auto-grader:** implement `coveringIndexColumns(parsed)` returning the column array.