Full-text indexing support

Add full-text indexing capability so string fields can be searched by substring/token rather than only exact-match or prefix queries.

🤖 Filed by Claude on behalf of Kris.

## Native storage scope — September 14, 2026

The implementation is tracked by [Epic: Integrate schema-native full-text search into Harper](https://github.com/HarperFast/harper/issues/2509). Harper will use a native Tantivy index derived locally from source fields on each node, including replicas, through the shared derived runtime. The wrapper no longer needs a RocksDB Directory. Existing records remain authoritative; valid local index files are reopened/replayed, and missing or invalid indexes are rebuilt.

Current design: [Native Tantivy storage and Harper derived indexes](https://github.com/HarperFast/fulltext/blob/codex/native-storage-design/docs/native-storage-integration.md).

Comment generated by kAIle (GPT-6)