Verify: DDL generation

[debba]

Description

Verify that DDL generation produces correct SQL against a real Db2 instance.

Methods to verify

• get_create_table_sql — generate CREATE TABLE SQL
• get_add_column_sql — generate ALTER TABLE ... ADD COLUMN SQL
• get_alter_column_sql — generate ALTER TABLE ... ALTER/RENAME COLUMN SQL
• get_create_index_sql — generate CREATE INDEX SQL
• get_create_foreign_key_sql — generate FK DDL

Acceptance criteria

• Generated SQL is valid and executable on Db2
• All supported data types and options are handled correctly

Part of the WIP verification checklist in README.