Add support for checkpoints.

[ioquatix]

Summary

Adds support for checkpoints: absolute snapshots of state, in contrast to regular migrations which are deltas.

• A checkpoint migration is identified by the checkpoint marker in its name, for example 20260101000000-checkpoint.rb.
• Migrate::Controller#migrate!(checkpoint:):
  • checkpoint: true — apply the most recent checkpoint, then only the migrations which sort after it.
  • checkpoint: "name" — apply the checkpoint with that name, then the migrations which sort after it. Raises RuntimeError if it does not exist.
  • checkpoint: nil (default) — full replay: every non-checkpoint migration is applied and checkpoint files are skipped, yielding the same final state as before.
• Migrate::Migration#checkpoint mirrors #migrate and delegates to the migrator's checkpoint method.
• Migrate::Provider#checkpoint raises NotImplementedError until implementations override it.
• The bake migrate task accepts a checkpoint: keyword and passes it through.

Checkpoints are optional, so existing setups are unaffected.

Tests

Covers: full replay (checkpoints skipped), latest checkpoint, pinned checkpoint, checkpoint not found, and FakeCheckpointMigrator receiving the checkpoint call with the expected arguments.