No Python API to prune frag-reuse generations, so deferred-remap users accumulate read-path debt forever

Deferred-remap compaction (defer_index_remap=true) appends a new generation to the __lance_frag_reuse system index on every commit. Rust already has a complete and race-safe pruner, cleanup_frag_reuse_index in rust/lance/src/dataset/index/frag_reuse.rs, which drops every generation that all indexes have caught up to. However, this function is not exposed in Python and is never invoked automatically, so a Python pipeline that defers remaps accumulates generations without bound. Because the frag-reuse index is loaded on every vector index open, the open-time cost grows with compaction history even after every index has been rebuilt and no generation is needed anymore.

I propose exposing the existing function as LanceDataset.cleanup_frag_reuse_index() so it can be scheduled as a periodic maintenance call alongside cleanup_old_versions. A patch is available that follows the migrate_manifest_paths_v2 wrapper pattern and includes a test asserting that __lance_frag_reuse reports zero versions once all indexes have caught up and the cleanup runs.

Start with cleanup_frag_reuse_index in rust/lance/src/dataset/index/frag_reuse.rs, then compare the Python wrapper for migrate_manifest_paths_v2. Add the LanceDataset cleanup entry point and a test verifying that __lance_frag_reuse reports zero versions after all indexes have caught up and cleanup runs.