Minor.Migration System Features

Add a first-class migration system under remix/data-table/migrations with:

• createMigration(...) and timestamp-based migration loading
• chainable column builders plus schema APIs for create, alter, drop, and index work
• createMigrationRunner(adapter, migrations) for up, down, status, and dryRun
• migration journaling, checksum tracking, and optional Node loading from remix/data-table/migrations/node

Migration callbacks now use split handles: { db, schema }.

• db is the immediate data runtime (query/create/update/delete/exec/transaction)
• schema owns migration operations like createTable, alterTable, plan, and introspection

Migration-time DDL, DML, and introspection now share the same transaction token when migration transactions are enabled. In dryRun, schema introspection (schema.hasTable / schema.hasColumn) reads live adapter/database state and does not simulate pending dry-run operations.

Add public subpath exports for migrations, Node migration loading, SQL helpers, operators, and SQL builders. SQL compilation stays adapter-owned, while shared SQL compiler helpers remain available from remix/data-table/sql-helpers.

@remix-run/data-table/migrations no longer exports a separate Database type alias. Migration callbacks still receive context.db as the main Database runtime, so if you need the type directly, import Database from @remix-run/data-table instead.