Add experimental Harper-backed Fulltext runtime

[kylebernhardy]

Summary

• add an experimental @harperfast/fulltext/harper entry point that runs Tantivy through a synchronous Harper-owned key-value view
• persist the derived-index cursor in the same Tantivy commit generation and reload the reader before publish() resolves
• enforce owner-worker writer exclusivity, bounded transport admission, callback fencing, and safe worker teardown
• preserve the native Tantivy entry point while keeping RocksDB and rocksdb-js out of this package
• document ambiguous publish outcomes by making committedPayload unavailable until reopen

This is the narrow Fulltext-side runtime slice for Phase 0: prove the Harper-backed Tantivy storage and derived-index path #7. It intentionally does not add Harper schema declarations, Table.search, non-owner readers, generation retirement, or release enablement.

Verification

• npm run check
• cargo clippy --locked --all-targets --features node-api -- -D warnings
• cargo clippy --locked --all-targets --features host-storage -- -D warnings
• local plan review by three independent reviewers
• committed implementation review by Claude and direct Gemini CLI review; relevant findings resolved

Advances #7

Comment generated by kAIle (GPT-5)