Implement native Tantivy backend and performance baseline

[kylebernhardy]

Closes #16.

Summary

• adds the standalone @harperfast/fulltext/native entry point backed by Tantivy MmapDirectory
• keeps the indexing and query engine generic over Tantivy Directory so the RocksDB adapter uses the same code path
• adds versioned packed mutation/search protocols, one bounded writer actor per index, parallel search workers, stable errors, deterministic close, crash recovery, and backend identity validation
• coordinates all indices owned by one Node environment through one bounded napi-rs cleanup hook
• adds BM25 search with approximate totals by default and explicit exact counts
• adds release-mode product-catalog benchmarks with commit cadence, durable throughput, queue/engine timing, same-concurrency exact-total comparison, search percentiles, index size, and sampled RSS
• documents that Harper releases select only the RocksDB backend

Verification

• npm run check
• 36 Rust tests passed
• 21 active Node/package tests passed; 3 rocksdb-js integration tests remain environment-gated
• search-while-write, poison-close/reopen, abrupt-exit, single- and multi-index worker teardown, overload, identity, and package-artifact regressions covered
• all GitHub checks pass on macOS, Windows, Ubuntu/Node 22.18, Ubuntu/Node 24, supply-chain/native linkage, and benchmark smoke
• Claude full and delta reviews: all major findings resolved; final verdict COMMENTS
• Gemini CLI was attempted directly but its configured API key was rejected, so no Gemini verdict was available

Local release benchmark

Apple arm64, Node 24.16.0, Tantivy 0.26.1:

• 100k documents, commits every 25k: 93.2k durable docs/s; approximate p99 0.80 ms at concurrency 4; sampled peak RSS 268 MB
• 1M documents, commits every 100k: 162.6k durable docs/s; approximate p99 6.20 ms at concurrency 8; exact-count p99 8.82 ms at concurrency 1; sampled peak RSS 684 MB; 30.4 MB index

These are development baselines, not the 100M-document or Harper p99 release qualification. Fixed-host release history and native-versus-Rocks comparisons remain in #15.

Comment generated by kAIle (Codex GPT-5)