checksum drift detection and repeatable migrations

[jaemk]

Checksum drift detection and repeatable migrations

Two related features built on the checksum bookkeeping column: enforcing that an applied
migration's SQL has not changed, and its inverse, migrations that re-run when it does.

Drift detection

• Verify that every already-applied migration still matches its recorded checksum before an up
  run applies anything, aborting with Error::ChecksumMismatch. The check runs even when
  nothing is pending, and skips rows where either side is null (programmatic or legacy rows).
• Opt out with --allow-checksum-mismatch (Migrator::allow_checksum_mismatch), independent
  of the existing --allow-unknown-tags / --allow-out-of-order.

Repeatable migrations

• Declare one with a -- migrant:repeatable directive in the up-SQL or the repeatable()
  builder method; Migratable::is_repeatable() reports it.
• Re-run whenever the up-SQL checksum changes, instead of applying exactly once. Unchanged
  migrations are skipped.
• Run them after all pending versioned migrations, in definition order, at most once per run.
• Keep one bookkeeping row per tag, updated in place, so recorded application order is
  preserved.
• Exempt them from the drift, unknown-tag, and out-of-order checks. The available migration set
  is authoritative on kind for a tag it still defines; the is_repeatable column decides only
  for tags no longer in the set.
• Forward-only: a down run never selects them, and registering one with no checksum or with a
  down direction is an Error::Migration.
• Add --rerun-repeatable (Migrator::rerun_repeatable) to re-run them regardless of
  checksum, since editing the file is otherwise the only trigger.
• Warn from redo naming the repeatable migrations it will not revert, since it targets the
  most recent versioned migration instead.
• Add migrant new --repeatable, report repeatable/stale from status and list, and add
  Report::repeatable_tags().

Breaking changes

• __migrant_migrations gains an is_repeatable column. This folds into the existing
  unreleased bookkeeping schema change; see the CHANGELOG for the upgrade SQL.
• down.sql is now optional for file-discovered migrations. A migration with no down file is a
  no-op in that direction.
• NewMigration::down_path() returns Option<&Path>.

Coverage

Unit and integration tests for both features, run against sqlite plus postgres and mysql via
migrant_lib/test.sh. Specs in spec/checksum-drift-detection.md and
spec/repeatable-migrations.md; guide pages and both changelogs updated.