bookkeeping schema, migration ordering, and 1.0 API changes

[jaemk]

Breaking changes ahead of the 1.0 release.

Bookkeeping and migrations

• add id, checksum, and applied_at columns to __migrant_migrations; record a sha256 of each migration's up SQL (None for FnMigration) and revert the last-applied migration by recorded order on Down
• error on an applied tag missing from the defined set or an out-of-order apply; opt out with Migrator::allow_unknown_tags / allow_out_of_order and the CLI --allow-unknown-tags / --allow-out-of-order flags on apply and redo
• sort discovered migrations by (stamp, tag) for a deterministic total order, so same-second migrations no longer resolve nondeterministically

CLI

• apply all pending migrations on migrant apply; add --step N for a bounded run and remove --all from apply (redo keeps it). --down stays single-step by default
• print the created file paths from migrant new

Library API

• rename migrant_lib::new to create_migration returning NewMigration; move list to migrant_lib::cli::list
• make settings-builder database_path / migration_location infallible (validation deferred to build()), and reject non-UTF-8 paths in init to match
• take Direction by value in Migratable::description and cli::edit; seal MigratableClone; add Migratable::checksum
• add the sha2 dependency

Upgrade

The __migrant_migrations schema change is a breaking on-disk change. See CHANGELOG.md for the table upgrade, or re-create the table.

Tests

• library unit, tests/sqlite.rs, and tests/migrant.rs green; cargo clippy --all-targets and cargo fmt --all --check clean
• postgres and mysql verified via bash migrant_lib/test.sh (throwaway docker), including the __migrant_migrations schema assertions (checksum recorded, applied_at not null, order by id)