UNION clause

[rfrneo4j]

1. AST (ast.rs)
   [ ] Add UnionClause struct (contains left and right queries)
   [ ] Add UnionAllClause struct (or use a flag)
   [ ] Consider: UNION might need a different Query structure (multiple queries combined)

2. Clause enum (clauses.rs)
   [ ] Add Union(UnionClause) variant
   [ ] Add UnionAll(UnionClause) variant (or use a flag)

3. Parser functions (clauses.rs)
   [ ] Add union_clause() parser function
   [ ] Add union_all_clause() parser function
   [ ] Parse two complete queries separated by UNION

4. Clause dispatcher (clauses.rs)
   [ ] Add UNION to clause() function's alt() parser
   [ ] Add UNION ALL to clause() function's alt() parser

5. Query assembly (clauses.rs)
   [ ] Handle UNION in parse_query() - may need different structure
   [ ] UNION combines two queries, so Query struct might need unions: Vec

6. Clause order validation (clauses.rs)
   [ ] Add UNION to validate_clause_order() state machine
   [ ] UNION must connect two complete queries (both must have RETURN)
   [ ] UNION can only appear between complete queries

7. Clause name function (clauses.rs)
   [ ] Add Clause::Union() => "UNION" to clause_name()
   [ ] Add Clause::UnionAll() => "UNION ALL" to clause_name()

8. Validation extraction (validation.rs)
   [ ] Add UNION clause extraction in extract_query_elements()
   [ ] Validate both sides of UNION have compatible RETURN columns

9. Tests (clauses.rs)
   [ ] Add parsing tests (UNION, UNION ALL)
   [ ] Add clause order validation tests
   [ ] Add validation extraction tests
   [ ] Test RETURN column compatibility

10. Documentation
    [ ] Update PARSER_INTERNALS.md
    [ ] Update CHANGELOG.md

11. Special considerations
    [ ] UNION requires rethinking Query structure (combines two queries)
    [ ] Both sides of UNION must have RETURN clauses
    [ ] RETURN columns must be compatible (same number, compatible types)