Implement bounded native execution and one-writer-per-index coordination

## Outcome

Run native indexing, merging, publication, and search with bounded admission and deterministic ownership.

## Acceptance criteria

- Exactly one Tantivy IndexWriter per canonical native directory/index/generation; independent indexes can progress in parallel.
- Use bounded native writer/commit lanes and search execution outside the JS event loop and shared libuv pool.
- Enforce aggregate memory, resident writer, native thread, merge, queue and search budgets; current per-index settings are not proof of process-wide bounds.
- Count retained native state during worker loss and preserve control/query completion under saturation.
- Distinguish queued cancellation from started non-cancelable native operations; panic/failure poisons the affected generation.
- Keep Harper runner election and epoch fencing in Harper; native directory locks remain the physical writer guard.
- No synchronous host-storage handoff or blocked-storage-thread pool is required for native MmapDirectory.

## 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)