cleanup_old_versions can delete files held by long-running readers

Description

A Dataset handle at an older version (via checkout_version) has no way to signal cleanup_old_versions that it's in use. If cleanup runs while a reader is mid-scan, the manifest and data files for that version can be deleted out from under it, and the scan fails with object-store NotFound errors.

Reproduce

1. Open a dataset and checkout_version(N) where N is older than lance.auto_cleanup.older_than.
2. Start a scan. While the scan is running, run cleanup_old_versions with a before_version that includes N (or let auto-cleanup fire on the next commit).
3. The scan fails mid-stream with NotFound from the object store.

Current workarounds

Create a temporary tag before the read and delete it after. Works, but every reader has to cooperate and it gets awkward on auto-cleanup-per-commit deployments — cleanup runs on every write.

What I'd want

A short-lived, renewable signal that a reader can publish for the version it's using: cleanup treats the version as retained while the signal is live, and a crashed reader doesn't pin the version forever. Something like an advisory lease with a TTL.

Happy to put up a PR if this direction sounds reasonable.

Start with the cleanup_old_versions and checkout_version entry points described in the issue, then trace how an older-version Dataset scan accesses its manifest and data files. Define how a short-lived renewable reader signal should interact with before_version and automatic cleanup, and verify that an active scan remains readable while expired or crashed-reader signals do not retain versions forever.