Epic: Integrate native Tantivy persistence with Harper derived indexes

## Outcome

Provide native checkpoint and lifecycle primitives that Harper's existing derived runtime can use without a custom byte-storage provider.

## Acceptance criteria

- Prove the native checkpoint and real Harper path in issue #7.
- Publish the opaque source checkpoint in the same Tantivy commit as the index state and expose validated readback after reopen.
- Support one writer per canonical native directory and safe reader/native-task lifetime across close and owner loss.
- Keep generic native APIs in this repository; Harper owns source cursors, replay, generation selection, paths, replication, and activation.
- Support independent indexes under measured process-wide resource budgets.
- Retire the hosted Directory, transport and custom KV reclamation work; no rocksdb-js changes are required.

## Design

[Native Tantivy storage and Harper derived indexes](https://github.com/HarperFast/fulltext/blob/codex/native-storage-design/docs/native-storage-integration.md). Native Tantivy files are the only delivery target for standalone use and Harper. Proposed APIs and integration gates are not claims of completed implementation.

Comment generated by kAIle (GPT-6)