Optimize RocksDB prefix scans for persistent forest leaves

The persistent forest stores leaf keys as `LineageId || leaf_index`. `entries(lineage)` scans leaves by that 32-byte lineage prefix, but `LEAVES_CF` does not set a RocksDB prefix extractor.

RocksDB can use prefix seek optimizations when a prefix extractor is configured:

- [Prefix Seek](https://github.com/facebook/rocksdb/wiki/Prefix-Seek)
- [Iterator: Prefix Iterating](https://github.com/facebook/rocksdb/wiki/Iterator#prefix-iterating)
- [Tuning Guide: Prefix database on flash storage](https://github.com/facebook/rocksdb/wiki/RocksDB-Tuning-Guide#prefix-database-on-flash-storage)

## Suggested change

Set a fixed 32-byte prefix extractor for `LEAVES_CF`, matching `LineageId`.

Benchmark `entries(lineage)` before and after. Only add prefix bloom or hash index tuning if the benchmark supports it.