Complete ANSI SELECT clauses

[piraz]

Remaining ANSI SELECT extensions

This follow-up starts after ticket #6 completes the ANSI SELECT core.

Scope

Implement and test the remaining ANSI SELECT structures, including:

• subqueries and derived table sources;
• common table expressions (CTEs);
• UNION and UNION ALL;
• INTERSECT and INTERSECT ALL;
• EXCEPT and EXCEPT ALL;
• other ANSI SELECT constructs identified after the core scope is complete.

Keep the semantic model dialect-neutral and add compiler/traversal tests for each feature. Dialect-specific rendering and support checks remain separate.

Boundary

DISTINCT, GROUP BY, HAVING, and FULL OUTER JOIN belong to ticket #6 and are not duplicated here.