Minor.Ddl Migration Contract

Add first-class migration execution support to the postgres adapter. It now compiles and executes DataMigrationOperation plans for remix/data-table/migrations, including create/alter/drop table and index flows, migration journal writes, and adapter-managed migration locking.

Normal reads/writes continue through execute(...), while migration/DDL work runs through migrate(...).

SQL compilation remains adapter-owned and can share helpers from remix/data-table/sql-helpers.