[M2] RocksDB persistent index storage

[Fosurero]

Description

Add on-disk persistence so the optimizer index survives restarts and can handle datasets larger than RAM.

Tasks

• Add rocksdb dependency
• Serialize/deserialize inverted index to RocksDB
• Incremental index updates (append-only)
• Startup: load existing index from disk
• Configurable data directory
• Benchmark: disk-backed vs in-memory query performance

Acceptance Criteria

• Index 1M blocks, restart process, query without re-indexing
• Query latency < 2× in-memory baseline