fix: add schema-preserving table rebuilds

[gsmlg]

Summary

• add Ecto.Adapters.Turso.Migration.rebuild_table!/3 with explicit target schema and identifier-only copy mappings
• pin rebuilds to one idle connection with transactional create/copy/drop/rename, FK state restoration, object recreation, sequence preservation, and mandatory integrity checks
• reject unsafe transaction, qualified-name, hidden-column, PK-mapping, and AUTOINCREMENT changes before destructive steps
• document reversible Ecto up/0 and down/0 usage and point unsupported modify/3 calls to the helper

Validation

• mix do --app ex_turso cmd mix test — 49 tests, 0 failures
• mix do --app ex_turso cmd mix compile --warnings-as-errors
• mix format --check-formatted
• mix credo --strict
• mix hex.build package-content verification

Fixes #82