feat: add rollback/down migrations support

[roor0]

Summary

Adds automatic down migration generation and runtime rollback support to Drizzle.

Down SQL Generation (drizzle-kit)

• drizzle-kit generate now produces a down.sql file alongside each migration.sql
• Down SQL is generated by running the schema diff in reverse with rename-aware resolvers
• Renames produce proper RENAME statements instead of destructive DROP/CREATE
• Supported for all dialects: PostgreSQL, MySQL, SQLite, LibSQL, SingleStore, CockroachDB
• Bundled migrations (Expo, Durable SQLite, OP-SQLite) gain a downMigrations record in migrations.js

Runtime Rollback (drizzle-orm)

• New rollback(db, config, steps?) export on every driver migrator
• Reads the N most-recently-applied migrations from the tracking table
• Executes their down SQL in reverse order within a transaction
• Removes the corresponding tracking rows
• Supports all 30+ driver adapters

Files Changed

• drizzle-kit: generate-common.ts, generate-down-helpers.ts (new), all generate-*.ts files
• drizzle-orm: migrator.ts, all dialect dialect.ts files, all driver migrator.ts files
• Tests: down-sql.test.ts, down-helpers.test.ts

Test plan

• Unit tests for writeResult down SQL file generation
• Unit tests for embeddedMigrations down SQL bundling
• Unit tests for withCapture rename capture
• Unit tests for makeInverseResolver rename inversion
• TypeScript compilation passes (tsc --noEmit)
• Integration tests with actual database rollback (future)